message: core: don't call on_discovery with dummy system
  These are check to prevent us from calling on_discovery if we're
  actually just a dummy system and should wait until something real is
  connected.
diff: @@ -322,6 +322,14 @@ void DronecodeSDKImpl::register_on_discover(const DronecodeSDK::event_callback_t
  if (callback) {
  for (auto const &connected_system : _systems) {
  + // Ignore dummy system with system ID 0.
  + if (connected_system.first == 0) {
  + continue;
  + }
  + // Ignore system if UUID is not initialized yet.
  + if (connected_system.second->get_uuid() == 0) {
  + continue;
  + }
  callback(connected_system.second->get_uuid());
  }
  }
  