Fix Typo and Added null check in RetirementDefectionTracker

[IllianiCBT]

Corrected the typo in the resource key "recentPromotion" and added a null check for the last promotion date before calculating the months difference. This ensures the application does not encounter a silent null pointer exception.

Closes #5235

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 10.50%. Comparing base (bce5f63) to head (580dec2).
Report is 68 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5242      +/-   ##
============================================
- Coverage     10.52%   10.50%   -0.02%     
+ Complexity     6047     6039       -8     
============================================
  Files           957      957              
  Lines        134523   134609      +86     
  Branches      19545    19535      -10     
============================================
- Hits          14154    14147       -7     
- Misses       119018   119106      +88     
- Partials       1351     1356       +5     

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