UniqueTogetherValidator does not comply to Database standards

[gabn88]

Checklist

• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.

If you have a database unique constraint that includes a nullable field, DRF will still raise an 'field_A and field_B must make a unique set' validationerror when you try to post a new nullable field. Whereas on database level every NULL is considered unique.

Could be fixed maybe here in the code:

django-rest-framework/rest_framework/validators.py

Line 123 in 63063da

def filter_queryset(self, attrs, queryset, serializer):