message: fix mission_raw_plugin  response for donwload mission
diff: @@ -173,6 +173,10 @@ void MissionRawImpl::process_mission_count(const mavlink_message_t &message)
  _mission_download.state = MissionDownload::State::REQUEST_ITEM;
  + if (mission_count.count == 0) {
  + _mission_download.state = MissionDownload::State::SHOULD_ACK;
  + }
  +
  // Let's just add this to the queue, this way we don't block the receive thread
  // and let go of the mutex as well.
  _parent->register_timeout_handler(
  