Fix component model initialization while cube fitting

[rosteen]

This fixes the broadcast error when trying to initialize model components with Cube Fit enabled by removing flux_nan_mask from _initialize_model_component and instead handling NaNs properly in the initializers themselves.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.61%. Comparing base (ca8c8ec) to head (87aaf06).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3857      +/-   ##
==========================================
- Coverage   87.63%   87.61%   -0.02%     
==========================================
  Files         188      188              
  Lines       25703    25747      +44     
==========================================
+ Hits        22525    22559      +34     
- Misses       3178     3188      +10     

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