Fix null filtering for *Choice filters

[rpkilby]

This is an update that should supersede/close #639. Thanks @michael-mri for an initial implementation and some test cases.

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TODO:

• Add test for lazy choice callable evaluation. See ChoiceFilter callable choices #710 and ChoiceFilter: don't call callable choices immediately #744.

In short, the null option handling is moved to a set of custom fields and out of the ChoiceFilter. This code largely emulates and extends what's done by forms.ModelChoiceField already. eg, the ChoiceIterator basically copies the behavior of ModelChoiceIterator and adds empty_label handling to ChoiceField & MultipleChoiceField. Null choice handling is just an extension of the empty choice.

Notes:

• The ChoiceIteratorMixin is probably the most confusing bit to reason about, due to how ModelChoiceFields can set both queryset and choices.
• ModelChoiceField & ModelMultipleChoiceField both validate the incoming values as part of the queryset. This check essentially has to be bypassed for null_values.
• Unlike, ModelChoiceField, ChoiceField doesn't already support empty_label (added in the PR).
• MultipleChoiceField explicitly sets empty_label to none, since it's an option that never makes sense. This is the same behavior as forms.ModelMultipleChoiceField.
• The auto-generation explicitly disables the null choice for non-nullable fields.
• There is no null_label handling for ManyToManyFields in FILTER_FOR_DBFIELD_DEFAULTS since the null option is not present/implicit.
• Since the empty option handling is now provided by the field, custom filters can create their own choices without having to explicitly handle these options (eg, AllValuesFilter, which should be fixed).

[codecov-io]

Codecov Report

Merging #680 into develop will increase coverage by 0.09%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #680      +/-   ##
===========================================
+ Coverage    98.11%   98.21%   +0.09%     
===========================================
  Files           15       15              
  Lines         1117     1179      +62     
===========================================
+ Hits          1096     1158      +62     
  Misses          21       21

Impacted Files	Coverage Δ
django_filters/filterset.py	100% <ø> (ø)	⬆️
django_filters/fields.py	100% <100%> (ø)	⬆️
django_filters/filters.py	98.44% <100%> (-0.04%)	⬇️

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[rpkilby]

Hi @carltongibson - finally got around to writing the test for #710. This PR should be ready for review now.

[carltongibson]

OK. Great thanks. I'll review all, and roll a new release shorty*

[*] ≈Before end of Summer

[rpkilby]

rebased

[carltongibson]

Lovely. Lets have it! 👍