Fix RatisConsensus client==null exception when stopping datanode

[OneSizeFitsQuorum]

Fix

[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

Attention: Patch coverage is 50.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 39.42%. Comparing base (1a0d4c6) to head (13045d3).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
...g/apache/iotdb/consensus/ratis/RatisConsensus.java	50.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #14578      +/-   ##
============================================
- Coverage     39.42%   39.42%   -0.01%     
- Complexity      185      193       +8     
============================================
  Files          4341     4342       +1     
  Lines        275376   275485     +109     
  Branches      33637    33637              
============================================
+ Hits         108570   108604      +34     
- Misses       166806   166881      +75     

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