### 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
StringName from_name = p_library == StringName() ? p_name : StringName(String(p_library) + "/" + String(p_name));
	StringName to_name = p_library == StringName() ? p_to_name : StringName(String(p_library) + "/" + String(p_to_name));
	if (!animation_set.has(from_name)) {
		return; 
	}
	_animation_set_cache_update();
	_rename_animation(from_name, to_name);
}
void AnimationMixer::_animation_changed(const StringName &p_name) {
	_clear_caches();
}
void AnimationMixer::_set_active(bool p_active) {
	
}
void AnimationMixer::_remove_animation(const StringName &p_name) {
	
}
void AnimationMixer::_rename_animation(const StringName &p_from_name, const StringName &p_to_name) {
	
}
TypedArray<StringName> AnimationMixer::_get_animation_library_list() const {
	TypedArray<StringName> ret;
	for (const AnimationLibraryData &lib : animation_libraries) {
		ret.push_back(lib.name);
	}
	return ret;
}
void AnimationMixer::get_animation_library_list(List<StringName> *p_libraries) const {