Don't hold space for companion windows if there aren't any.

[cbeer]

Before:

After:

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (a08aa6a) 92.50% compared to head (2514469) 92.50%.
Report is 14 commits behind head on mui5.

Files	Patch %	Lines
src/components/ScrollIndicatedDialogContent.js	50.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             mui5    #3867      +/-   ##
==========================================
- Coverage   92.50%   92.50%   -0.01%     
==========================================
  Files         203      203              
  Lines        3576     3575       -1     
==========================================
- Hits         3308     3307       -1     
  Misses        268      268              

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