Support configurable nullable field type/value

[Ksauder]

Pydantic has an experimental MISSING sentinel that allows a type annotation to declare an attribute optional and not default null. This is extremely useful if you are trying to make efficient data transfer and have models with a lot of optional data fields.

I have a PR up on the upstream to allow the configuration of what the null type and value for a nullable field in a DjangoModel.

Depending on movement there, would this be a change you would be interested in here?

[pmdevita]

Sure, I'd be interested. It seems like you're getting some traction on Ninja, if it goes through we can merge it here when we next sync upstream. Otherwise, you can submit a PR here.