[fix](fe) Skip dropped columns in follower stats sync

[yujun777]

What problem does this PR solve?

FollowerColumnSender drains queued column references on follower FEs and syncs the columns that still need analysis to the master. A queued column can become stale after DDL changes. If the table still exists but the queued column has been dropped, table.getColumn(column.colName) returns null and the sender throws a NullPointerException while reading the type.

This patch skips dropped columns before checking the column type, so the daemon does not emit periodic ERROR logs and can continue processing the remaining queued columns.

Issue Number: close #xxx

Release note

None

Check List (For Author)

• Test: Unit Test
  • org.apache.doris.statistics.FollowerColumnSenderTest
• Behavior changed: No
• Does this need documentation: No

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[yujun777]

run buildall

[hello-stephen]

TPC-H: Total hot run time: 31615 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpch-tools
Tpch sf100 test result on commit ff333368ee9336fbfb7f89b17c74aeeb4a85a01e, data reload: false
------ Round 1 ----------------------------------
orders	Doris	NULL	NULL	0	0	0	NULL	0	NULL	NULL	2023-12-26 18:27:23	2023-12-26 18:42:55	NULL	utf-8	NULL	NULL ============================================
q1	17718	4056	4003	4003
q2	q3	10757	1408	824	824
q4	4711	475	347	347
q5	7845	2298	2114	2114
q6	396	174	135	135
q7	928	780	659	659
q8	9556	1815	1664	1664
q9	7115	5003	5000	5000
q10	6444	2303	1912	1912
q11	430	273	249	249
q12	690	415	290	290
q13	18226	3427	2803	2803
q14	267	258	245	245
q15	q16	835	801	716	716
q17	1020	938	956	938
q18	6962	5777	5621	5621
q19	1189	1366	1130	1130
q20	502	414	266	266
q21	5769	2616	2393	2393
q22	429	359	306	306
Total cold run time: 101789 ms
Total hot run time: 31615 ms
----- Round 2, with runtime_filter_mode=off -----
orders	Doris	NULL	NULL	150000000	42	6422171781	NULL	22778155	NULL	NULL	2023-12-26 18:27:23	2023-12-26 18:42:55	NULL	utf-8	NULL	NULL ============================================
q1	4374	4406	4324	4324
q2	q3	4541	4913	4324	4324
q4	2076	2173	1373	1373
q5	4426	4305	5129	4305
q6	244	196	138	138
q7	1991	1871	1584	1584
q8	2438	2122	2243	2122
q9	8020	8026	7987	7987
q10	4903	4789	4319	4319
q11	559	410	418	410
q12	741	783	533	533
q13	3331	3651	2949	2949
q14	299	314	284	284
q15	q16	717	756	670	670
q17	1397	1384	1356	1356
q18	8122	7279	6732	6732
q19	1118	1132	1079	1079
q20	2225	2234	1954	1954
q21	5296	4596	4449	4449
q22	528	472	413	413
Total cold run time: 57346 ms
Total hot run time: 51305 ms

[hello-stephen]

TPC-DS: Total hot run time: 171821 ms

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/tpcds-tools
TPC-DS sf100 test result on commit ff333368ee9336fbfb7f89b17c74aeeb4a85a01e, data reload: false
query5	4330	658	531	531
query6	329	227	203	203
query7	4224	606	346	346
query8	325	241	235	235
query9	8754	4064	4076	4064
query10	452	340	302	302
query11	5808	2595	2226	2226
query12	182	134	131	131
query13	1282	641	469	469
query14	6141	5492	5200	5200
query14_1	4533	4523	4506	4506
query15	218	208	188	188
query16	1041	482	475	475
query17	1179	747	619	619
query18	2728	491	362	362
query19	228	207	174	174
query20	139	135	129	129
query21	217	145	121	121
query22	13681	13621	13338	13338
query23	17347	16601	16289	16289
query23_1	16343	16404	16367	16367
query24	7437	1807	1379	1379
query24_1	1383	1333	1347	1333
query25	587	502	444	444
query26	1297	315	183	183
query27	2685	603	343	343
query28	4465	2040	2040	2040
query29	964	615	500	500
query30	289	238	196	196
query31	1139	1084	947	947
query32	90	75	75	75
query33	525	356	296	296
query34	1184	1199	651	651
query35	778	801	711	711
query36	1385	1374	1257	1257
query37	156	105	96	96
query38	3218	3170	3104	3104
query39	922	976	891	891
query39_1	879	862	862	862
query40	236	143	128	128
query41	65	63	62	62
query42	114	113	108	108
query43	330	329	295	295
query44 query45	211	211	202	202
query46	1070	1174	757	757
query47	2367	2371	2243	2243
query48	419	395	300	300
query49	624	496	379	379
query50	1028	356	263	263
query51	4400	4459	4290	4290
query52	106	106	94	94
query53	256	279	210	210
query54	308	278	259	259
query55	94	91	87	87
query56	327	304	297	297
query57	1431	1430	1314	1314
query58	298	281	266	266
query59	1604	1679	1486	1486
query60	314	323	318	318
query61	154	158	157	157
query62	705	656	593	593
query63	243	207	211	207
query64	2377	821	656	656
query65 query66	1650	482	360	360
query67	29673	29621	29573	29573
query68 query69	474	352	308	308
query70	1054	1039	1008	1008
query71	297	258	266	258
query72	2930	2714	2462	2462
query73	859	815	464	464
query74	5094	4998	4777	4777
query75	2720	2601	2263	2263
query76	2287	1125	773	773
query77	408	399	347	347
query78	12431	12451	11823	11823
query79	1484	1027	749	749
query80	637	536	453	453
query81	458	285	242	242
query82	1385	161	122	122
query83	350	281	254	254
query84	258	149	113	113
query85	923	562	460	460
query86	400	363	308	308
query87	3447	3393	3268	3268
query88	3596	2731	2724	2724
query89	451	401	342	342
query90	1897	186	187	186
query91	181	175	141	141
query92	77	81	79	79
query93	1475	1409	890	890
query94	556	367	315	315
query95	693	476	359	359
query96	1109	816	345	345
query97	2736	2738	2625	2625
query98	231	222	229	222
query99	1183	1154	1037	1037
Total cold run time: 254313 ms
Total hot run time: 171821 ms

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 0.00% (0/3) 🎉
Increment coverage report
Complete coverage report

[yujun777]

run external

[yujun777]

run external

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 0.00% (0/3) 🎉
Increment coverage report
Complete coverage report

[hello-stephen]

FE Regression Coverage Report

Increment line coverage 0.00% (0/3) 🎉
Increment coverage report
Complete coverage report

[morrySnow]

/review

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[github-actions]

Review result: no blocking issues found.

Critical checkpoint conclusions:

• Goal and correctness: The PR fixes the follower stats sync NPE by skipping stale queued columns whose metadata has disappeared, while still processing remaining queued columns.
• Scope: The change is small and focused on the failing null dereference path, with no unrelated GitHub PR files.
• Concurrency and lifecycle: No new shared state, locking, thread lifecycle, or static initialization behavior is introduced. The existing queue-draining daemon behavior is unchanged.
• Compatibility and persistence: No thrift schema, storage format, edit log, or rolling-upgrade compatibility impact.
• Parallel paths: Existing table column-index pair implementations already skip missing/unsupported columns; this change covers the earlier direct getColumn(...).getType() access in the follower sender path.
• Error handling: The stale dropped-column case is now handled by skipping the item instead of letting a NullPointerException abort the send round.
• Test coverage: A focused FE unit test was added for dropped-column queue entries, and CI reports FE UT and CheckStyle passing. I did not run additional local tests.
• Observability and performance: No new observability needed; the added null check is trivial and avoids periodic error logs.
• Data correctness: No transaction, visibility-version, delete-bitmap, or data write path is affected.

User focus: No additional user-provided review focus was specified.