### 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
track_cache = p_backup->get_data();
	_blend_apply();
	track_cache = HashMap<NodePath, AnimationMixer::TrackCache *>();
	cache_valid = false;
}
#ifdef TOOLS_ENABLED
Ref<AnimatedValuesBackup> AnimationMixer::apply_reset(bool p_user_initiated) {
	if (!p_user_initiated && dummy) {
		return Ref<AnimatedValuesBackup>();
	}
	ERR_FAIL_COND_V(!can_apply_reset(), Ref<AnimatedValuesBackup>());
	Ref<Animation> reset_anim = animation_set[SceneStringNames::get_singleton()->RESET].animation;
	ERR_FAIL_COND_V(reset_anim.is_null(), Ref<AnimatedValuesBackup>());
	Ref<AnimatedValuesBackup> backup_current = make_backup();
	if (p_user_initiated) {
		EditorUndoRedoManager *ur = EditorUndoRedoManager::get_singleton();
		ur->create_action(TTR("Animation Apply Reset"));
		ur->add_do_method(this, "_reset");
		ur->add_undo_method(this, "_restore", backup_current);
		ur->commit_action();
	} else {
		reset();
	}
	return backup_current;
}
#endif 
void AnimationMixer::_node_removed(Node *p_node) {
	_clear_caches();
}
void AnimationMixer::_notification(int p_what) {