'Return the first field from self.touched_fields that is null on the
mi object'

def test_fields(self, mi):

    for key in self.touched_fields:
        if key.startswith(u'identifier:'):
            key = key.partition(u':')[(-1)]
            if (key == u'amazon'):
                if (self.domain != u'com'):
                    key += (u'_' + self.domain)
            if (not mi.has_identifier(key)):
                return (u'identifier:    ' + key)
        elif mi.is_null(key):
            return key
