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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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())