'Returns ``None`` if the metadata URL doesn\'t return JSON content.'

def test_not_json(self):

    listening = webserver_for_test(self, url_path=('/' + '/'.join(METADATA_RELATIVE_PATH)), response_content=random_name(self))
    connecting = listening.addCallback((lambda port: deferToThread(metadata_from_service, metadata_service_endpoint=next(((a.host, a.port) for a in [port.getHost()])))))
    def check(result):
        self.assertIs(None, result)
    checking = connecting.addCallback(check)
    return checking
