### Godot version

v4.2.rc.custom_build [d6a1db2b0]

### System information

Godot v4.2.rc (d6a1db2b0) - Windows 10.0.22621 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3090 Ti (NVIDIA; 31.0.15.4592) - AMD Ryzen 9 7950X 16-Core Processor (32 Threads)

### Issue description

`AnimatedValuesBackup::get_data()` can crash as the internal data map can have nullptr values if any of the tracks were METHOD or ANIMATION types, as `AnimatedValuesBackup::get_cache_copy()` defines that these types of tracks do not get valid copies.

The resulting crash is therefore a null pointer exception.

### Steps to reproduce

1. Open a project with these type of animation tracks.
2. Save the project.

### Minimal reproduction project

https://github.com/godotengine/godot-demo-projects/tree/master/3d/platformer
ERR_FAIL_COND(!can_apply_reset());
	Ref<Animation> reset_anim = animation_set[SceneStringNames::get_singleton()->RESET].animation;
	ERR_FAIL_COND(reset_anim.is_null());
	Node *root_node_object = get_node_or_null(root_node);
	ERR_FAIL_NULL(root_node_object);
	AnimationPlayer *aux_player = memnew(AnimationPlayer);
	root_node_object->add_child(aux_player);
	Ref<AnimationLibrary> al;
	al.instantiate();
	al->add_animation(SceneStringNames::get_singleton()->RESET, reset_anim);
	aux_player->set_reset_on_save_enabled(false);
	aux_player->set_root_node(aux_player->get_path_to(root_node_object));
	aux_player->add_animation_library("", al);
	aux_player->set_assigned_animation(SceneStringNames::get_singleton()->RESET);
	aux_player->seek(0.0f, true);
	aux_player->queue_free();
}
void AnimationMixer::restore(const Ref<AnimatedValuesBackup> &p_backup) {