'__exact=value is transformed to __isnull=True if Field.get_prep_value()
converts value to None.'

def test_custom_field_none_rhs(self):

    season = Season.objects.create(year=2012, nulled_text_field=None)
    self.assertTrue(Season.objects.filter(pk=season.pk, nulled_text_field__isnull=True))
    self.assertTrue(Season.objects.filter(pk=season.pk, nulled_text_field=''))
