### Godot version

4.0 beta 17

### System information

Windows 10

### Issue description

Create a minimal multiplayer project with a scene that contains a MultiplayerSpawner which is setup to spawn things into this scene, but it has no pre-configured "auto spawn list". From code, before starting the host or client, invoke add_spawnable_scene() with a scene, in this case res://client.tscn. Start the host and then join with the client, nothing is being spawned for the client. It seems that a MultiplayerSpawner that initially has an empty "auto spawn list" is not properly initialized and adding a scene to it programmatically fails.

### Steps to reproduce

Please use the attached project. It contains three different scenes you can launch. The default scene called "world-dynamic-bug.tscn" is the one that shows the issue. Make sure to run two instances. On the first, press "Host". The window will go blank. On the second, press "Join" and that window will go blank too.
At this point, client and server have connected and if you inspect the scene tree for both you will see the server one is fine, but the client one lacks both "res://client.tscn" instances. You will also see that the MultiplayerSpawner for both did get this client scene added programmatically.
Now stop both instances, and run the "world-static-works.tscn" scene. This one does not programmatically add the "client" scene, but it is configured statically in the world scene. You will see that this works just fine.
Finally, try "world-dynamic-workaround", which adds one non-existing spawnable scene called "res://nonsense" (which does not even exist in the project). If you run this scene, you will also see it works.

As a bit of background, I ran "wireshark" on all scenarios and in the non-working scenario I don't see a packet that looks like this:
<img width="1120" alt="image" src="https://user-images.githubusercontent.com/446570/217062195-375894fc-7e89-48e8-a7d0-e0d63d44d472.png">
My guess is something is not correctly initialized (upfront or when later adding the first scene).

### Minimal reproduction project

[multiplayer-loading.zip](https://github.com/godotengine/godot/files/10668256/multiplayer-loading.zip)

return spawn_path;
}
void MultiplayerSpawner::set_spawn_path(const NodePath &p_path) {
	spawn_path = p_path;
	_update_spawn_node();
}
void MultiplayerSpawner::_track(Node *p_node, const Variant &p_argument, int p_scene_id) {
	ObjectID oid = p_node->get_instance_id();
	if (!tracked_nodes.has(oid)) {
		tracked_nodes[oid] = SpawnInfo(p_argument.duplicate(true), p_scene_id);
		p_node->connect(SceneStringNames::get_singleton()->tree_exiting, callable_mp(this, &MultiplayerSpawner::_node_exit).bind(p_node->get_instance_id()), CONNECT_ONE_SHOT);
		_spawn_notify(p_node->get_instance_id());
	}
}
void MultiplayerSpawner::_spawn_notify(ObjectID p_id) {
	get_multiplayer()->object_configuration_add(ObjectDB::get_instance(p_id), this);
}
void MultiplayerSpawner::_node_exit(ObjectID p_id) {
	Node *node = Object::cast_to<Node>(ObjectDB::get_instance(p_id));
	ERR_FAIL_COND(!node);
	if (tracked_nodes.has(p_id)) {
		tracked_nodes.erase(p_id);
		get_multiplayer()->object_configuration_remove(node, this);
	}
}
int MultiplayerSpawner::find_spawnable_scene_index_from_path(const String &p_scene) const {