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