message: fix SIGSEGV in is_connected() when no system is connected
  Fixes an issue in which if an application (e.g., offboard_velocity) is started
  before the simulator is started, a segmentation fault is caused because it
  tries to dereference a null pointer.
diff: @@ -255,6 +255,10 @@ bool DronecodeSDKImpl::is_connected() const
  {
  std::lock_guard<std::recursive_mutex> lock(_systems_mutex);
  + if (_systems.empty()) {
  + return false;
  + }
  +
  return _systems.begin()->second->is_connected();
  }
  