Fixed the NPE when validating legacy sink

[Caideyipi]

Description

As the title said.

---

This PR has:

• been self-reviewed.
  • concurrent read
  • concurrent write
  • concurrent read and write
• added documentation for new or modified features or behaviors.
• added Javadocs for most classes and all non-trivial methods.
• added or updated version, license, or notice information
• added comments explaining the "why" and the intent of the code wherever would not be obvious
  for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold
  for code coverage.
• added integration tests.
• been tested in a test IoTDB cluster.

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Key changed/added classes (or packages if there are too many classes) in this PR

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 39.48%. Comparing base (e4ef4db) to head (a19d8d1).
⚠️ Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
...pipe/sink/protocol/legacy/IoTDBLegacyPipeSink.java	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #17153   +/-   ##
=========================================
  Coverage     39.47%   39.48%           
  Complexity      282      282           
=========================================
  Files          5098     5098           
  Lines        341273   341278    +5     
  Branches      43471    43472    +1     
=========================================
+ Hits         134714   134737   +23     
+ Misses       206559   206541   -18     

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