[fix](nereids) fix copy into fail when enable debug log

[sollhui]

What problem does this PR solve?

Introduced by #47194.

Fix copy into fail when enable debug log:

Cannot invoke "Object.toString()" because the return value of "org.apache.doris.nereids.trees.plans.commands.info.CopyFromDesc.getFileColumns()" is null

Release note

None

Check List (For Author)

• Test

  • Regression test
  • Unit Test
  • Manual test (add detailed scripts or steps below)
  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.
    • No code files have been changed.
    • Other reason

• Behavior changed:

  • No.
  • Yes.

• Does this need documentation?

  • No.
  • Yes.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[Thearas]

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?

[sollhui]

run buildall

[doris-robot]

ClickBench: Total hot run time: 30.16 s

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/clickbench-tools
ClickBench test result on commit 619deb81e6323f7746ffa942635ea5508fd06fcd, data reload: false
query1	0.06	0.05	0.05
query2	0.09	0.04	0.06
query3	0.25	0.08	0.08
query4	1.61	0.12	0.12
query5	0.28	0.27	0.26
query6	1.15	0.65	0.65
query7	0.03	0.03	0.03
query8	0.05	0.05	0.04
query9	0.62	0.54	0.50
query10	0.57	0.58	0.59
query11	0.17	0.12	0.12
query12	0.15	0.12	0.12
query13	0.63	0.62	0.63
query14	1.03	1.04	1.03
query15	0.87	0.86	0.85
query16	0.41	0.40	0.41
query17	1.05	1.03	1.01
query18	0.22	0.20	0.19
query19	1.94	1.81	1.78
query20	0.02	0.01	0.02
query21	15.45	0.93	0.58
query22	0.77	1.16	0.68
query23	14.92	1.39	0.62
query24	7.27	1.49	0.56
query25	0.45	0.21	0.07
query26	0.59	0.16	0.14
query27	0.07	0.06	0.05
query28	8.75	1.32	0.93
query29	12.62	3.91	3.27
query30	0.28	0.14	0.11
query31	2.83	0.59	0.39
query32	3.24	0.55	0.49
query33	3.14	3.07	3.19
query34	16.18	5.47	4.88
query35	4.91	4.92	4.97
query36	0.69	0.52	0.51
query37	0.10	0.08	0.08
query38	0.07	0.05	0.04
query39	0.04	0.03	0.03
query40	0.18	0.16	0.16
query41	0.09	0.04	0.03
query42	0.04	0.03	0.03
query43	0.04	0.04	0.04
Total cold run time: 103.92 s
Total hot run time: 30.16 s

[hello-stephen]

FE UT 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

[liaoxin01]

LGTM

[github-actions]

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

[github-actions]

PR approved by anyone and no changes requested.

[dataroaring]

LGTM