Fix potential NPE in tryGetFLushLock

[JackieTien97]

This pull request improves the robustness of the tryGetFLushLock method in DataRegion.java by handling cases where a TsFileProcessor is null, which indicates the file is in the process of closing. The new logic introduces a busy-wait loop to check for the file's closed status before proceeding, ensuring thread safety and correctness during concurrent file operations.

Concurrency and file closing handling:

• Added a check for tsFileProcessor == null to detect if a tsfile is being closed, and implemented a busy-wait loop that waits for the file to finish closing before proceeding. If the file closes within the wait time, the loop continues; otherwise, it clears the lock list and returns false to prevent unsafe operations. (iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/DataRegion.java)

[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 0% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.89%. Comparing base (54ca825) to head (f78997f).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
.../iotdb/db/storageengine/dataregion/DataRegion.java	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master   #16274   +/-   ##
=========================================
  Coverage     38.89%   38.89%           
  Complexity      198      198           
=========================================
  Files          4894     4894           
  Lines        320486   320494    +8     
  Branches      40533    40536    +3     
=========================================
+ Hits         124664   124671    +7     
- Misses       195822   195823    +1     

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