### 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
case Animation::TYPE_AUDIO: {
				TrackCacheAudio *t = static_cast<TrackCacheAudio *>(track);
				Node *asp = Object::cast_to<Node>(t->object);
				if (asp) {
					t->object->call(SNAME("set_stream"), Ref<AudioStream>());
				}
			} break;
			default: {
			} 
		}
	}
}
Ref<AnimatedValuesBackup> AnimationMixer::make_backup() {
	Ref<AnimatedValuesBackup> backup;
	backup.instantiate();
	Ref<Animation> reset_anim = animation_set[SceneStringNames::get_singleton()->RESET].animation;
	ERR_FAIL_COND_V(reset_anim.is_null(), Ref<AnimatedValuesBackup>());
	_blend_init();
	PlaybackInfo pi;
	pi.time = 0;
	pi.delta = 0;
	pi.seeked = true;
	pi.weight = 1.0;
	make_animation_instance(SceneStringNames::get_singleton()->RESET, pi);
	_build_backup_track_cache();
	backup->set_data(track_cache);
	clear_animation_instances();
	return backup;
}
void AnimationMixer::reset() {