### 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
if (resource.is_valid()) {
							track_bezier->object = resource.ptr();
						} else {
							track_bezier->object = child;
						}
						track_bezier->subpath = leftover_path;
						track_bezier->object_id = track_bezier->object->get_instance_id();
						track = track_bezier;
						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_bezier->init_value = (reset_anim->track_get_key_value(rt, 0).operator Array())[0];
							}
						}
					} break;
					case Animation::TYPE_AUDIO: {
						TrackCacheAudio *track_audio = memnew(TrackCacheAudio);
						track_audio->object = child;
						track_audio->object_id = track_audio->object->get_instance_id();
						track_audio->audio_stream.instantiate();
						track_audio->audio_stream->set_polyphony(audio_max_polyphony);
						track = track_audio;
					} break;
					case Animation::TYPE_ANIMATION: {
						TrackCacheAnimation *track_animation = memnew(TrackCacheAnimation);
						track_animation->object = child;
						track_animation->object_id = track_animation->object->get_instance_id();
						track = track_animation;
					} break;
					default: {