### 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
root_motion_position = Vector3(0, 0, 0);
	root_motion_rotation = Quaternion(0, 0, 0, 1);
	root_motion_scale = Vector3(0, 0, 0);
	root_motion_position_accumulator = Vector3(0, 0, 0);
	root_motion_rotation_accumulator = Quaternion(0, 0, 0, 1);
	root_motion_scale_accumulator = Vector3(1, 1, 1);
	if (!cache_valid) {
		if (!_update_caches()) {
			return;
		}
	}
	
	for (const KeyValue<NodePath, TrackCache *> &K : track_cache) {
		TrackCache *track = K.value;
		track->total_weight = 0.0;
		switch (track->type) {
			case Animation::TYPE_POSITION_3D: {
				TrackCacheTransform *t = static_cast<TrackCacheTransform *>(track);
				if (track->root_motion) {
					root_motion_cache.loc = Vector3(0, 0, 0);
					root_motion_cache.rot = Quaternion(0, 0, 0, 1);
					root_motion_cache.scale = Vector3(1, 1, 1);
				}
				t->loc = t->init_loc;
				t->rot = t->init_rot;
				t->scale = t->init_scale;