### 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
return editing;
}
void AnimationMixer::set_dummy(bool p_dummy) {
	dummy = p_dummy;
}
bool AnimationMixer::is_dummy() const {
	return dummy;
}
#endif 
void AnimationMixer::_clear_caches() {
	_init_root_motion_cache();
	_clear_audio_streams();
	_clear_playing_caches();
	for (KeyValue<NodePath, TrackCache *> &K : track_cache) {
		memdelete(K.value);
	}
	track_cache.clear();
	cache_valid = false;
	emit_signal(SNAME("caches_cleared"));
}
void AnimationMixer::_clear_audio_streams() {