message: core: never call a nullptr callback
diff: @@ -321,9 +321,11 @@ void DroneCoreImpl::register_on_discover(const DroneCore::event_callback_t callb
  {
  std::lock_guard<std::recursive_mutex> lock(_systems_mutex);
  + if (callback) {
  for (auto const &connected_system : _systems) {
  callback(connected_system.second->get_uuid());
  }
  + }
  _on_discover_callback = callback;
  }
  