Fix broken null handling logic

[villebro]

This PR started out as a two-liner in one file, but grew slightly due to unrelated issues in branch. Summary of all changes:

• Backport fix for regression in handle_nulls() introduced in Refactor NULL handling into method, disable for DECK.gl vizes #5106
• Backport unit test fix from Fix flaky unit test - remove 'Markup' object in data #5313
• Backport select parts from Move flake8-related packages deps to reqs-dev.txt #5460 of tox.ini and requirements-dev.txt to avoid false positive regression in current stable pylint (fires E1101 incorrectly in viz.py for missing members in geopy.point.Point)
• Fix a few linting errors

[mistercrunch]

Thanks for fixing this. LGTM apart from the build errors.

[codecov-io]

Codecov Report

Merging #5619 into 0.27 will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##             0.27    #5619      +/-   ##
==========================================
- Coverage   61.39%   61.28%   -0.11%     
==========================================
  Files         369      369              
  Lines       23536    23530       -6     
  Branches     2724     2720       -4     
==========================================
- Hits        14449    14421      -28     
- Misses       9075     9097      +22     
  Partials       12       12

Impacted Files	Coverage Δ
...t/assets/src/dashboard/reducers/getInitialState.js	0% <ø> (ø)	⬆️
...rset/assets/src/dashboard/actions/sliceEntities.js	32.25% <ø> (+1.95%)	⬆️
...c/dashboard/components/dnd/AddSliceDragPreview.jsx	61.9% <ø> (+2.81%)	⬆️
superset/utils.py	87.92% <ø> (-0.03%)	⬇️
superset/models/helpers.py	87.7% <100%> (-0.07%)	⬇️
superset/viz.py	79.98% <100%> (-1.69%)	⬇️

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[villebro]

Ready for review @mistercrunch, please see updated summary above.

[villebro]

ping @mistercrunch WRT the release today of 0.27.0, this PR was pending review for rc1.