### 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
ERR_CONTINUE_MSG(anim->track_get_key_count(i) == 0, "AnimationMixer: '" + String(E) + "', Value Track:  '" + String(path) + "' must have at least one key to cache for blending.");
						track_value->init_value = anim->track_get_key_value(i, 0);
						track_value->init_value.zero();
						
						if (has_reset_anim) {
							int rt = reset_anim->find_track(path, track_type);
							if (rt >= 0 && reset_anim->track_get_key_count(rt) > 0) {
								track_value->init_value = reset_anim->track_get_key_value(rt, 0);
							}
						}
					} break;
					case Animation::TYPE_POSITION_3D:
					case Animation::TYPE_ROTATION_3D:
					case Animation::TYPE_SCALE_3D: {
#ifndef _3D_DISABLED
						Node3D *node_3d = Object::cast_to<Node3D>(child);
						if (!node_3d) {
							ERR_PRINT("AnimationMixer: '" + String(E) + "', transform track does not point to Node3D:  '" + String(path) + "'.");
							continue;
						}
						TrackCacheTransform *track_xform = memnew(TrackCacheTransform);
						track_xform->type = Animation::TYPE_POSITION_3D;
						track_xform->node_3d = node_3d;
						track_xform->skeleton = nullptr;
						track_xform->bone_idx = -1;
						bool has_rest = false;
						if (path.get_subname_count() == 1 && Object::cast_to<Skeleton3D>(node_3d)) {
							Skeleton3D *sk = Object::cast_to<Skeleton3D>(node_3d);
							track_xform->skeleton = sk;