Fix Traceback from compound models when removing items

[soininen]

CompoundModelBase.handle_items_removed() now drops single models that become empty. This prevents those models from trying to access deleted entity classes.

Fixes #2483

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black
• Unit tests pass

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (624ec8e) 63.66% compared to head (466470c) 63.72%.

Files	Patch %	Lines
...olbox/spine_db_editor/mvcmodels/compound_models.py	84.21%	1 Missing and 2 partials ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           0.8-dev    #2490      +/-   ##
===========================================
+ Coverage    63.66%   63.72%   +0.05%     
===========================================
  Files          195      195              
  Lines        35487    35564      +77     
  Branches      4996     6039    +1043     
===========================================
+ Hits         22593    22663      +70     
+ Misses       11714    11708       -6     
- Partials      1180     1193      +13     

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