Scanner cannot handle heartbeat message with no results

When a RS scanner get a TIME_LIMIT_REACHED_MID_ROW state, they will stop scanning and send back what it has read to client and mark the message as a heartbeat message. If there is no cell has been read, it will be an empty response.

However, ClientScanner only handles the situation that the client gets an empty heartbeat and its cache is not empty. If the cache is empty too, it will be regarded as end-of-region and open a new scanner for next region.