Fix operator tests for transient null TsBlocks

[Caideyipi]

Summary

• add OperatorTestUtils.nextNonNull / lastNonNull for test-only consumption of operators whose next() may return transient null
• update Offset, series scan/aggregation, fill, concat, into, merge-sort, and last-cache operator tests to use the helper instead of directly dereferencing next() results
• keep tests that intentionally assert null or already validate null positions unchanged

Tests

• git diff --check
• mvn -pl iotdb-core/datanode spotless:check -Ddevelocity.off=true -DskipTests
• Not run to completion locally: targeted datanode JUnit commands fail before selected tests because this workspace cannot currently compile/testCompile datanode cleanly without a complete main reactor output

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.79%. Comparing base (66d0c07) to head (02f5bc3).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@## master #17838 +/- ##
============================================
- Coverage 40.98% 40.79% -0.20% - Complexity 2611 2613 +2 
============================================
Files 5188 5200 +12 Lines 351973 353687 +1714 Branches 45071 45252 +181 ============================================
+ Hits 144272 144292 +20 - Misses 207701 209395 +1694 

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[sonarqubecloud]

Quality Gate failed

Failed conditions
10.8% Duplication on New Code (required ≤ 5%)

See analysis details on SonarQube Cloud