Sends an empty request over the streaming pull RPC.

        This always sends over the stream, regardless of if
        ``self._UNARY_REQUESTS`` is set or not.

def heartbeat(self):
        """Sends an empty request over the streaming pull RPC.

        This always sends over the stream, regardless of if
        ``self._UNARY_REQUESTS`` is set or not.
        """
        if self._rpc is not None and self._rpc.is_active:
            self._rpc.send(types.StreamingPullRequest())