### 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
for (int i = 0; i < playing_audio_stream_players.size(); i++) {
		playing_audio_stream_players[i]->call(SNAME("stop"));
		playing_audio_stream_players[i]->call(SNAME("set_stream"), Ref<AudioStream>());
	}
	playing_audio_stream_players.clear();
}
void AnimationMixer::_clear_playing_caches() {
	for (const TrackCache *E : playing_caches) {
		if (ObjectDB::get_instance(E->object_id)) {
			E->object->call(SNAME("stop"));
		}
	}
	playing_caches.clear();
}
void AnimationMixer::_init_root_motion_cache() {
	root_motion_cache.loc = Vector3(0, 0, 0);
	root_motion_cache.rot = Quaternion(0, 0, 0, 1);
	root_motion_cache.scale = Vector3(1, 1, 1);
	root_motion_position = Vector3(0, 0, 0);
	root_motion_rotation = Quaternion(0, 0, 0, 1);
	root_motion_scale = Vector3(0, 0, 0);
	root_motion_position_accumulator = Vector3(0, 0, 0);
	root_motion_rotation_accumulator = Quaternion(0, 0, 0, 1);
	root_motion_scale_accumulator = Vector3(1, 1, 1);
}
bool AnimationMixer::_update_caches() {