message: core: Only call new callback with existing systems
  This should fix the logic so that we only notify the new/just added
  callback about existing systems but not the already saved callbacks.
  Without this, we would notify users every time a new subscriber is
  added.
diff: @@ -628,13 +628,12 @@ Mavsdk::NewSystemHandle
  MavsdkImpl::subscribe_on_new_system(const Mavsdk::NewSystemCallback& callback)
  {
  std::lock_guard<std::recursive_mutex> lock(_systems_mutex);
  - const bool is_first_subscription = _new_system_callbacks.empty();
  - const auto handle = _new_system_callbacks.subscribe(callback);
  - if (is_first_subscription && is_any_system_connected()) {
  - _new_system_callbacks.queue([this](const auto& func) { call_user_callback(func); });
  + if (callback != nullptr && is_any_system_connected()) {
  + call_user_callback([temp_callback = callback]() { temp_callback(); });
  }
  - return handle;
  +
  + return _new_system_callbacks.subscribe(callback);
  }
  void MavsdkImpl::unsubscribe_on_new_system(Mavsdk::NewSystemHandle handle)
  