mypy typing fixes: fixed implicit optionals

[dskrypa]

Fixed most implicit optionals to explicitly allow None as part of #151

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.66%. Comparing base (d1adf62) to head (5d7b430).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   97.66%   97.66%           
=======================================
  Files          52       52           
  Lines        6946     6946           
  Branches     1097     1097           
=======================================
  Hits         6784     6784           
  Misses        127      127           
  Partials       35       35           

Flag	Coverage Δ
unittests	97.66% <100.00%> (ø)

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