### 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
_process_animation(p_time);
}
void AnimationMixer::clear_caches() {
	_clear_caches();
}
void AnimationMixer::set_root_motion_track(const NodePath &p_track) {
	root_motion_track = p_track;
}
NodePath AnimationMixer::get_root_motion_track() const {
	return root_motion_track;
}
Vector3 AnimationMixer::get_root_motion_position() const {
	return root_motion_position;
}
Quaternion AnimationMixer::get_root_motion_rotation() const {
	return root_motion_rotation;
}
Vector3 AnimationMixer::get_root_motion_scale() const {
	return root_motion_scale;
}
Vector3 AnimationMixer::get_root_motion_position_accumulator() const {
	return root_motion_position_accumulator;
}
Quaternion AnimationMixer::get_root_motion_rotation_accumulator() const {
	return root_motion_rotation_accumulator;
}
Vector3 AnimationMixer::get_root_motion_scale_accumulator() const {
	return root_motion_scale_accumulator;
}
void AnimationMixer::set_reset_on_save_enabled(bool p_enabled) {