Browsable API forms should replace empty values with null for fields with null=True by default, like ModelForms do

[tkoft]

https://code.djangoproject.com/ticket/4136?cversion=0&cnum_hist=33

This was fixed in Django's ModelForms long ago, but seems to be an issue on DRF's browsable API. There's no way to pass in a null field in a form on the browsable API, only empty string, so fields with null=True should convert to null as well.