Handle None types

[tclose]

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Summary

• Handles None in field types, e.g. int | None instead of typing.Optional[int]

Checklist

• [x] I have added tests to cover my changes (if necessary)
• [x] I have updated documentation (if necessary)

added by @ghisvail Closes #744

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.71%. Comparing base (e52e32b) to head (a7ac191).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #742      +/-   ##
==========================================
+ Coverage   84.27%   91.71%   +7.43%     
==========================================
  Files          26       55      +29     
  Lines        5127    15681   +10554     
  Branches     1447     2792    +1345     
==========================================
+ Hits         4321    14382   +10061     
- Misses        800     1291     +491     
- Partials        6        8       +2     

Flag	Coverage Δ
unittests	91.71% <100.00%> (+7.43%)	:arrow_up:

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[ghisvail]

I did a superficial read of the diff and what I checked so far looked sensible 👍