### 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
reset_on_save = p_enabled;
}
bool AnimationMixer::is_reset_on_save_enabled() const {
	return reset_on_save;
}
bool AnimationMixer::can_apply_reset() const {
	return has_animation(SceneStringNames::get_singleton()->RESET);
}
void AnimationMixer::_build_backup_track_cache() {
	for (const KeyValue<NodePath, TrackCache *> &K : track_cache) {
		TrackCache *track = K.value;
		track->total_weight = 1.0;
		switch (track->type) {
			case Animation::TYPE_POSITION_3D: {
#ifndef _3D_DISABLED
				TrackCacheTransform *t = static_cast<TrackCacheTransform *>(track);
				if (t->root_motion) {
					
				} else if (t->skeleton && t->bone_idx >= 0) {
					if (t->loc_used) {
						t->loc = t->skeleton->get_bone_pose_position(t->bone_idx);
					}
					if (t->rot_used) {
						t->rot = t->skeleton->get_bone_pose_rotation(t->bone_idx);
					}
					if (t->scale_used) {
						t->scale = t->skeleton->get_bone_pose_scale(t->bone_idx);
					}