MXS-2158 Return true if update_gtids() succeeds, even if no data is returned

Previously, if the server had no gtid:s, the method would fail leading to
a confusing error message. This could even totally stop the monitor from working
if a recent server version (10.X) did not have any gtid events.

server/modules/monitor/mariadbmon/mariadbserver.hh

@@ -202,7 +202,7 @@ public:
      * Query gtid_current_pos and gtid_binlog_pos and save the values to the server.
      *
      * @param errmsg_out Where to store an error message if query fails. Can be null.
-     * @return True if successful
+     * @return True if query succeeded
      */
     bool update_gtids(std::string* errmsg_out = NULL);