AP_RCProtocol: send null pkts for FPort2

when we have no data to send, we should send an empty packet to ensure
the receiver schedules us for data as often as possible

libraries/AP_RCProtocol/AP_RCProtocol_FPort2.cpp

@@ -116,7 +116,11 @@ void AP_RCProtocol_FPort2::decode_downlink(const FPort2_Frame &frame)
      */
     if (!telem_data.available) {
         if (!AP_Frsky_Telem::get_telem_data(telem_data.frame, telem_data.appid, telem_data.data)) {
-            return;
+            // nothing to send, send a null frame. This ensures the
+            // receiver keeps requesting data from us at the full rate
+            telem_data.frame = 0x00; 
+            telem_data.appid = 0x00; 
+            telem_data.data = 0x00;
         }
         telem_data.available = true;
     }