message: mavlink_passthrough: support unregistering
  The API docs say that nullptr can be used to unregister. Therefore, we
  should not add nullptr to the mavlink handlers but unregister properly.
diff: @@ -38,8 +38,12 @@ MavlinkPassthrough::Result MavlinkPassthroughImpl::send_message(mavlink_message_
  void MavlinkPassthroughImpl::subscribe_message_async(
  uint16_t message_id, std::function<void(const mavlink_message_t &)> callback)
  {
  + if (callback == nullptr) {
  + _parent->unregister_mavlink_message_handler(message_id, this);
  + } else {
  _parent->register_mavlink_message_handler(message_id, callback, this);
  }
  +}
  uint8_t MavlinkPassthroughImpl::get_our_sysid() const
  {
  