Nullable source of ReadOnlyField

[jennydaman]

Describe the bug

I added a test for this bug to a fork. See jennydaman@c124c7a
To Reproduce

import uuid
from django.db import models
from rest_framework import serializers, viewsets
from tests import assert_schema, generate_schema
class Album(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
class Song(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    album = models.ForeignKey(Album, on_delete=models.CASCADE, null=True)
class SongSerializer(serializers.ModelSerializer):
    album_id = serializers.ReadOnlyField(source='album.id')
    class Meta:
        fields = ['id', 'album_id']
        model = Song
class SongModelViewset(viewsets.ModelViewSet):
    serializer_class = SongSerializer
    queryset = Song.objects.none()

Expected behavior

The album_id field should be nullable:

components:
  schemas:
    Song:
      type: object
      properties:
        album_id:
          type: string
          format: uuid
          readOnly: true
          nullable: true  # <-- expected to be in schema, actually missing

[jennydaman]

One down, 98 to go... similar bug occurs with HyperlinkedRelatedField

edit: to handle a case like

import uuid

from django.db import models
from rest_framework import serializers, viewsets

from tests import assert_schema, generate_schema


class Cubicle(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)


class Employee(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    cubicle = models.ForeignKey(Cubicle, on_delete=models.CASCADE, null=True)


class EmployeeSerializer(serializers.HyperlinkedModelSerializer):

    cubicle_url = serializers.HyperlinkedRelatedField(view_name='cubicle-detail', read_only=True)

    class Meta:
        fields = ['id', 'cubicle_url']
        model = Employee

In the example above, EmployeeSerializer.cubicle_url could be null but it is not explicitly created with allow_null=True. In this situation, drf-spectacular could figure this out using some magic involving django.urls.reverse and django.urls.resolve. Unfortunately I myself don't have the energy to contribute this fix (i.e. #1307 is ready to merge, it doesn't fix all related bugs but it does add something)