failing test: optional implicit M2M / QueryDict

[leifurhauks]

Description

Failing test that demonstrates a validation inconsistency -- the implicit ManyToMany field on a model serializer, with required=False, fails when the input data is a QueryDict and the ManyToMany field is omitted; but succeeds for a plain dict.

[xordoquy]

The Model's field needs to have blank=True. You'll notice that the serializer doesn't complain about the lack of the field. It complains about the field being blank.
At first sight, this is a side effect of forms that can't set a value to null.