Model _default_manager can be None

[Sébastien Diemer]

When using Model inheritance with an abstract model in between, it can happen that the _default_manager of the actual child class is None. In this case the ModelState.from_model function fails with the following error:

django/django/db/migrations/state.py", line 439, in from_model
    default_manager_name = force_text(model._default_manager.name)
AttributeError: 'NoneType' object has no attribute 'name'

The following modified test reproduces the error:

    @override_settings(TEST_SWAPPABLE_MODEL='migrations.SomeFakeModel')
    def test_create_swappable(self):
        """
        Tests making a ProjectState from an Apps with a swappable model
        """
        new_apps = Apps(['migrations'])

        class Base(models.Model):
            pass

        class AbstractAuthor(Base):
            class Meta:
                abstract = True

        class Author(AbstractAuthor):
            name = models.CharField(max_length=255)
            bio = models.TextField()
            age = models.IntegerField(blank=True, null=True)

            class Meta(AbstractAuthor.Meta):
                app_label = 'migrations'
                apps = new_apps
                swappable = 'TEST_SWAPPABLE_MODEL'

        author_state = ModelState.from_model(Author)
        self.assertEqual(author_state.app_label, 'migrations')
        self.assertEqual(author_state.name, 'Author')
        self.assertEqual([x for x, y in author_state.fields], ['id', 'name', 'bio', 'age'])
        self.assertEqual(author_state.fields[1][1].max_length, 255)
        self.assertEqual(author_state.fields[2][1].null, False)
        self.assertEqual(author_state.fields[3][1].null, True)
        self.assertEqual(author_state.options, {'abstract': False, 'swappable': 'TEST_SWAPPABLE_MODEL'})
        self.assertEqual(author_state.bases, (models.Model, ))
        self.assertEqual(author_state.managers, [])

Changing the following line in ModelState.from_model

        if hasattr(model, "_default_manager"):

to

       if getattr(model, "_default_manager", None):

is probably sufficient to fix the bug.