Avoid ChoiceField duplicate enum values for allow_null, allow_blank

[intgr]

When using ChoiceField(choices=[..., ''], allow_blank=True) then drf-spectacular would add another dupllicate '' value into the schema.

Same happens with ChoiceField(choices=[..., None], allow_null=True).

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (6e4180b) 98.57% compared to head (caf0a70) 98.57%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1085   +/-   ##
=======================================
  Coverage   98.57%   98.57%           
=======================================
  Files          68       68           
  Lines        8426     8436   +10     
=======================================
+ Hits         8306     8316   +10     
  Misses        120      120           

Files	Coverage Δ
drf_spectacular/plumbing.py	97.07% <100.00%> (ø)
tests/test_plumbing.py	100.00% <100.00%> (ø)

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[tfranzel]

Excellent PR. Thanks @intgr

This is definitely an improvement to prevent useless duplication. However, I think it is generally bad form to explicitly include '' or None in the choice list, since this ought to be covered by the serializer/model field parameters. I don't remember how DRF handles this internally without looking it up, but this here is most likely the right thing to do (in the schema context) in either case.