Fix folderManager NPE issue in WALNode when starting iotdb in a disk-full state

[OneSizeFitsQuorum]

This PR addresses an NPE issue in WALNode creation that occurs when IoTDB starts with a full disk. When the disk is full at startup, the FolderManager initialization in the constructor fails, leaving it null. If disk space is later freed, subsequent WAL node creation attempts fail with NPE because the folderManager is still null.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 39.05%. Comparing base (f4d628a) to head (f6b1b7d). :warning: Report is 25 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #16869      +/-   ##
============================================
+ Coverage     38.87%   39.05%   +0.17%     
  Complexity      207      207              
============================================
  Files          5021     5021              
  Lines        333052   333150      +98     
  Branches      42389    42390       +1     
============================================
+ Hits         129487   130119     +632     
+ Misses       203565   203031     -534     

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[sonarqubecloud[bot]]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

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

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