message: issue#609: only send heartbeat after discovering vehicles
diff: @@ -304,7 +304,9 @@ void SystemImpl::system_thread()
  while (!_should_exit) {
  if (_time.elapsed_since_s(last_time) >= SystemImpl::_HEARTBEAT_SEND_INTERVAL_S) {
  + if (_parent.is_connected()) {
  send_heartbeat();
  + }
  last_time = _time.steady_time();
  }
  