Interpret empty VIDEO_CACHE_DIR as None

[ejm714]

If VIDEO_CACHE_DIR gets set to an empty string or 0, it will be interpreted as True for caching. This PR adds logic so that empty strings or 0s in the environment variable will be interpreted as no caching is desired.

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[github-actions]

🚀 Deployed on https://deploy-preview-245--silly-keller-664934.netlify.app

[pjbull]

LGTM!

[codecov-commenter]

Codecov Report

Merging #245 (4b18335) into master (00851f4) will decrease coverage by 0.2%.
The diff coverage is 100.0%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #245     +/-   ##
========================================
- Coverage    87.0%   86.7%   -0.3%     
========================================
  Files          28      28             
  Lines        1962    1936     -26     
========================================
- Hits         1707    1680     -27     
- Misses        255     256      +1     

Impacted Files	Coverage Δ
zamba/data/video.py	81.0% <100.0%> (-0.6%)	⬇️
zamba/version.py	80.0% <0.0%> (-20.0%)	⬇️
zamba/models/densepose/config.py	92.2% <0.0%> (-0.2%)	⬇️
zamba/models/config.py	96.8% <0.0%> (-0.2%)	⬇️
zamba/models/model_manager.py	84.2% <0.0%> (-0.1%)	⬇️
zamba/models/utils.py	100.0% <0.0%> (ø)