### 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
Variant res;
	if (GDVIRTUAL_CALL(_post_process_key_value, p_anim, p_track, p_value, const_cast<Object *>(p_object), p_object_idx, res)) {
		return res;
	}
	return _post_process_key_value(p_anim, p_track, p_value, p_object, p_object_idx);
}
Variant AnimationMixer::_post_process_key_value(const Ref<Animation> &p_anim, int p_track, Variant p_value, const Object *p_object, int p_object_idx) {
	switch (p_anim->track_get_type(p_track)) {
#ifndef _3D_DISABLED
		case Animation::TYPE_POSITION_3D: {
			if (p_object_idx >= 0) {
				const Skeleton3D *skel = Object::cast_to<Skeleton3D>(p_object);
				return Vector3(p_value) * skel->get_motion_scale();
			}
			return p_value;
		} break;
#endif 
		default: {
		} break;
	}
	return p_value;
}
void AnimationMixer::_blend_init() {