allow video_size = None in settings.py

[dsschult]

A small fix to allow video_size = None, which is the way to disable video resizing.

[codecov]

Codecov Report

Merging #435 (126fce5) into main (7ba5f7b) will increase coverage by 0.04%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##             main     #435      +/-   ##
==========================================
+ Coverage   86.48%   86.53%   +0.04%     
==========================================
  Files          22       22              
  Lines        1828     1834       +6     
==========================================
+ Hits         1581     1587       +6     
  Misses        247      247              

Impacted Files	Coverage Δ
sigal/gallery.py	89.44% <87.50%> (-0.10%)	⬇️
sigal/settings.py	97.91% <100.00%> (+0.04%)	⬆️
sigal/image.py	91.66% <0.00%> (+0.43%)	⬆️

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[saimn]

Thanks!