When a __prepared statement__ has no body in the query the backend responds with an `emptyQuery` message but never with a `commandComplete` or `errorResponse` message. The client was hanging forever waiting for one of the other two expected messages. The server was hanging forever waiting for the client to respond with a `sync` message. This change has the client send the required `sync` on receipt of an `emptyQuery` message when the query is a prepared statement. Fixes#822
