[hotfix] avoid npe when checking mirror server set assignment

[walterddr]

fix NPE introduce in #11578 (review)

[codecov-commenter]

Codecov Report

Merging #11915 (7c62044) into master (abad6c8) will decrease coverage by 0.03%.
Report is 3 commits behind head on master.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##             master   #11915      +/-   ##
============================================
- Coverage     61.50%   61.47%   -0.03%     
  Complexity     1147     1147              
============================================
  Files          2378     2378              
  Lines        128844   128851       +7     
  Branches      19925    19926       +1     
============================================
- Hits          79242    79209      -33     
- Misses        43858    43924      +66     
+ Partials       5744     5718      -26     

Flag	Coverage Δ
custom-integration1	<0.01% <0.00%> (ø)
integration	<0.01% <0.00%> (ø)
integration1	<0.01% <0.00%> (ø)
integration2	0.00% <0.00%> (ø)
java-11	34.73% <33.33%> (-26.71%)	⬇️
java-21	61.34% <33.33%> (+0.02%)	⬆️
skip-bytebuffers-false	61.44% <33.33%> (-0.04%)	⬇️
skip-bytebuffers-true	61.28% <33.33%> (+0.01%)	⬆️
temurin	61.47% <33.33%> (-0.03%)	⬇️
unittests	61.46% <33.33%> (-0.03%)	⬇️
unittests1	46.65% <33.33%> (+0.03%)	⬆️
unittests2	27.68% <0.00%> (-0.06%)	⬇️

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Files	Coverage Δ
...ery/planner/physical/MailboxAssignmentVisitor.java	85.18% <100.00%> (ø)
...mmon/assignment/InstanceAssignmentConfigUtils.java	0.00% <0.00%> (ø)

... and 26 files with indirect coverage changes

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