fix: Fix bug in stability test bypass

[dkachuma]

Fixes a bug introduced in a previous PR. When the mixture is a pure component, we skip the stability test and mark it as single phase. However the incipient phase composition wasn't being assigned leading to a crash down the line. This PR fixes that.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 58.74%. Comparing base (9d92895) to head (117f956).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ompositional/models/NegativeTwoPhaseFlashModel.hpp	75.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3783      +/-   ##
===========================================
- Coverage    58.74%   58.74%   -0.01%     
===========================================
  Files         1278     1278              
  Lines       110244   110246       +2     
===========================================
  Hits         64766    64766              
- Misses       45478    45480       +2     

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