fix: related filters with bogus data

[dpgaspar]

Description

Fixes issue when a user chooses an empty value from a dropdown for a related field then filter apply get's "__None" and this raises an exception on PG and other engines.

ADDITIONAL INFORMATION

• Has associated issue: Superset Show Detailed Error Message Instead of Custom Error Message apache/superset#16663
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

Merging #1695 (31e5555) into master (1bfedc2) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1695      +/-   ##
==========================================
+ Coverage   76.55%   76.67%   +0.12%     
==========================================
  Files          55       55              
  Lines        8061     8091      +30     
==========================================
+ Hits         6171     6204      +33     
+ Misses       1890     1887       -3     

Flag	Coverage Δ
python	76.67% <100.00%> (+0.12%)	⬆️

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Impacted Files	Coverage Δ
flask_appbuilder/exceptions.py	100.00% <100.00%> (ø)
flask_appbuilder/models/sqla/filters.py	80.83% <100.00%> (+4.17%)	⬆️
flask_appbuilder/views.py	66.11% <100.00%> (+0.42%)	⬆️

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