'Transforms are used for __exact=None.'

def test_exact_none_transform(self):

    Season.objects.create(year=1, nulled_text_field='not    null')
    self.assertFalse(Season.objects.filter(nulled_text_field__isnull=True))
    self.assertTrue(Season.objects.filter(nulled_text_field__nulled__isnull=True))
    self.assertTrue(Season.objects.filter(nulled_text_field__nulled__exact=None))
    self.assertTrue(Season.objects.filter(nulled_text_field__nulled=None))
