### 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
int bone_idx = sk->find_bone(path.get_subname(0));
							if (bone_idx != -1) {
								has_rest = true;
								track_xform->bone_idx = bone_idx;
								Transform3D rest = sk->get_bone_rest(bone_idx);
								track_xform->init_loc = rest.origin;
								track_xform->init_rot = rest.basis.get_rotation_quaternion();
								track_xform->init_scale = rest.basis.get_scale();
							}
						}
						track_xform->object = node_3d;
						track_xform->object_id = track_xform->object->get_instance_id();
						track = track_xform;
						switch (track_type) {
							case Animation::TYPE_POSITION_3D: {
								track_xform->loc_used = true;
							} break;
							case Animation::TYPE_ROTATION_3D: {
								track_xform->rot_used = true;
							} break;
							case Animation::TYPE_SCALE_3D: {
								track_xform->scale_used = true;
							} break;
							default: {
							}
						}
						
						if (has_reset_anim && !has_rest) {
							int rt = reset_anim->find_track(path, track_type);
							if (rt >= 0 && reset_anim->track_get_key_count(rt) > 0) {