Do not make Vega width/height responsive if fixed value is set

[philippjfr]

Fixes #5918

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (39ec575) 82.41% compared to head (1ec3162) 84.24%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5940      +/-   ##
==========================================
+ Coverage   82.41%   84.24%   +1.82%     
==========================================
  Files         291      291              
  Lines       43055    43068      +13     
==========================================
+ Hits        35485    36281     +796     
+ Misses       7570     6787     -783     

Flag	Coverage Δ
ui-tests	41.06% <25.00%> (+2.44%)	⬆️
unitexamples-tests	72.23% <100.00%> (+0.28%)	⬆️

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