'Check 200 is returned when sort_key is null disk_format
Check 200 is returned when sort_key is disk_format and
disk_format is null for specified marker'

def test_get_index_null_disk_format(self):

    UUID6 = _gen_uuid()
    extra_fixture = self.get_fixture(id=UUID6, disk_format=None, size=19)
    db_api.image_create(self.context, extra_fixture)
    self.get_api_response_ext(http.OK, url=('/images?sort_key=disk_format&marker=%s' % UUID6))
