More strict `.update` validation for non-nullable fields

[UnknownPlatypus]

Blocked by #3317

Bug report

As said in the comment below,MyModel.objects.all().update(id=None) should raise an error.
At runtime, it would be an IntegrityError: (1048, "Column 'id' cannot be null")

It affects:

• Field(primary_key=True)
• Field(null=False)

-   case: autofield_can_be_set_to_none
    main: |
        from myapp.models import MyModel, MyModelExplicitPK
        m = MyModel()
        m.id = 3
        m.id = None
        m2 = MyModel(id=None)
        MyModel.objects.create(id=None)
        MyModel.objects.all().update(id=None) # Should give an error since there's a not-null constraint

        def foo(a: int) -> bool:
          return True
        m2 = MyModel()
        foo(m2.id)

        # At runtime, this would be an error, unless m.save() was called to populate the `id` field.
        # but the plugin cannot catch this.
        foo(m.id)

        exp = MyModelExplicitPK()
        exp.id = 3
        exp.id = None
    installed_apps:
        - myapp
    files:
        -   path: myapp/__init__.py
        -   path: myapp/models.py
            content: |
                from django.db import models
                class MyModel(models.Model):
                    pass
                class MyModelExplicitPK(models.Model):
                    id = models.AutoField(primary_key=True)