Improve support for null atttibute values

[alexmv]

Rather than have these throw an assert at runtime, it is more useful to have them act as if the attribute did not exist.

[Alir3z4]

Some syntax sugar request to simplify the code readability.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7ba8431) 97.24% compared to head (3a37da4) 97.22%.

❗ Current head 3a37da4 differs from pull request most recent head 66dd5ba. Consider uploading reports for the commit 66dd5ba to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
- Coverage   97.24%   97.22%   -0.02%     
==========================================
  Files          11       11              
  Lines        1124     1118       -6     
==========================================
- Hits         1093     1087       -6     
  Misses         31       31              

Flag	Coverage Δ
unittests-3.10	97.22% <100.00%> (-0.02%)	⬇️
unittests-3.11	97.22% <100.00%> (-0.02%)	⬇️
unittests-3.12	97.22% <100.00%> (-0.02%)	⬇️
unittests-3.8	97.22% <100.00%> (-0.02%)	⬇️
unittests-3.9	97.22% <100.00%> (-0.02%)	⬇️

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[Alir3z4]

@alexmv Thanks for the fix, it's a pretty good catch.
I've left some comments, please let me know your feedback.

There are 2 files in conflicts as well as I've merged a big outstanding pull request #392

[alexmv]

Rebased and repushed. As I commented above, the if "x" in thing and thing["x"] is not None pattern is apparently necessary for mypy type narrowing.

[Alir3z4]

Thanks for the fix.
In the coming days, I'll cut a release and the change will be available on PyPI.