__empty__ choice raise AssertionError: Invalid nullable case

[gabn88]

Describe the bug
I'd like to add empty as choice on a nullable field, see: https://docs.djangoproject.com/en/5.1/ref/models/fields/#enumeration-types (at the bottom of the paragraph). However AssertionError: Invalid nullable case is then raised on scheme generation. I noticed this error is also raised when overriding the choices and adding a (None, 'unknown') tuple to the choices.

To Reproduce
Add

 __empty__ = 'unknown'

OR

        extra_kwargs = {'default': {'choices': list(SomeTextChoices.choices) + [(None, 'unknown')]}}

to the choices

Expected behavior
I would expect the field to have null as a choice.

I have tried all other methods to make the (read_only) field nullable, but this seems impossible.

What have I tried:
Add allow_null=True, allow_blank=True, required=False, amongst others.

I do have
"ENUM_ADD_EXPLICIT_BLANK_NULL_CHOICE": False,

because otherwise I loose a lot of Enums (they become simply "string", not enum, in the scheme generation).

I have also noticed that upgrading to 0.28.0 also made me lose a lot of read-only Enums, so I'm still on 0.27.2.

[tfranzel]

OAS 3.1 is not as battle-tested as the rest. Using a official feature like __empty__ should not assert, so that is definitely a bug.

Regarding your other statement:

I have also noticed that upgrading to 0.28.0 also made me lose a lot of read-only Enums, so I'm still on 0.27.2.

https://github.com/tfranzel/drf-spectacular/compare/0.28.0..0.27.2

I can only see one change that might have impacted enum generation: #1240

What does "lose" mean? Can you give an example? Is this a regression?