[rtemodel] Fix nullptr exception in RteCallback::ExpandString

[grasci-arm]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 63.04%. Comparing base (b555fda) to head (9637f38).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1474      +/-   ##
==========================================
+ Coverage   63.02%   63.04%   +0.01%     
==========================================
  Files         161      161              
  Lines       30531    30542      +11     
  Branches    18677    18682       +5     
==========================================
+ Hits        19243    19254      +11     
- Misses       7503     7526      +23     
+ Partials     3785     3762      -23     

Flag	Coverage Δ
buildmgr-cov	74.12% <ø> (ø)
packchk-cov	65.54% <ø> (ø)
projmgr-cov	80.11% <81.81%> (+0.02%)	⬆️

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Files	Coverage Δ
tools/projmgr/test/src/ProjMgrUnitTests.cpp	79.10% <81.81%> (+<0.01%)	⬆️

... and 11 files with indirect coverage changes