core: prevent invalid duplicate acks (#2424)

It turns out we respond with two acks to a MAV_REQUEST_MESSAGE when we
should only respond once with the successful one.

Alternatively, we could first grab all the results and then do some
magic to determine which one is the best ack but that's a bit too magic
and complex, so I'm not doing that yet.

src/mavsdk/plugins/telemetry_server/telemetry_server_impl.cpp

@@ -83,20 +83,25 @@ void TelemetryServerImpl::init()
                 case MAVLINK_MSG_ID_HOME_POSITION:
                     if (_maybe_home) {
                         if (_send_home()) {
-                            return _server_component_impl->make_command_ack_message(
-                                command, MAV_RESULT::MAV_RESULT_ACCEPTED);
+                            return std::optional<mavlink_command_ack_t>{
+                                _server_component_impl->make_command_ack_message(
+                                    command, MAV_RESULT::MAV_RESULT_ACCEPTED)};
                         } else {
-                            return _server_component_impl->make_command_ack_message(
-                                command, MAV_RESULT::MAV_RESULT_FAILED);
+                            return std::optional<mavlink_command_ack_t>{
+                                _server_component_impl->make_command_ack_message(
+                                    command, MAV_RESULT::MAV_RESULT_FAILED)};
                         }
                     } else {
-                        return _server_component_impl->make_command_ack_message(
-                            command, MAV_RESULT::MAV_RESULT_DENIED);
+                        return std::optional<mavlink_command_ack_t>{
+                            _server_component_impl->make_command_ack_message(
+                                command, MAV_RESULT::MAV_RESULT_DENIED)};
                     }
 
                 default:
-                    return _server_component_impl->make_command_ack_message(
-                        command, MAV_RESULT::MAV_RESULT_DENIED);
+                    // Let's hope someone else answers and keep silent. In an ideal world we would
+                    // explicitly deny all the ones that we ought to answer but haven't implemented
+                    // yet.
+                    return std::optional<mavlink_command_ack_t>{};
             }
         },
         this);