GCS_MAVLink: Do not compare delegate to NULL

Use the simpler "if (delegate_name)" since it allows simpler
implementation in the class, i.e. the bool operator rather than having
to compare to another object.

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -840,7 +840,7 @@ GCS_MAVLINK::update(run_cli_fn run_cli)
     {
         uint8_t c = comm_receive_ch(chan);
 
-        if (run_cli != NULL) {
+        if (run_cli) {
             /* allow CLI to be started by hitting enter 3 times, if no
              *  heartbeat packets have been received */
             if ((mavlink_active==0) && (hal.scheduler->millis() - _cli_timeout) < 20000 &&