Don't publish "null" node input topic

get_ansi_seq() here can return null, don't pass that value to mqtt.publish()
which gets converted to a string.

exec/mqtt_spy.js

@@ -117,6 +117,7 @@ while(!js.terminated) {
 				else
 					key = get_ansi_seq();
 			}
+			if(key)
 				mqtt.publish(in_topic, key);
 		}
 	}