### 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
switch (p_what) {
		case NOTIFICATION_ENTER_TREE: {
			if (!processing) {
				set_physics_process_internal(false);
				set_process_internal(false);
			}
			_clear_caches();
		} break;
		case NOTIFICATION_INTERNAL_PROCESS: {
			if (active && callback_mode_process == ANIMATION_CALLBACK_MODE_PROCESS_IDLE) {
				_process_animation(get_process_delta_time());
			}
		} break;
		case NOTIFICATION_INTERNAL_PHYSICS_PROCESS: {
			if (active && callback_mode_process == ANIMATION_CALLBACK_MODE_PROCESS_PHYSICS) {
				_process_animation(get_physics_process_delta_time());
			}
		} break;
		case NOTIFICATION_EXIT_TREE: {
			_clear_caches();
		} break;
	}
}
void AnimationMixer::_bind_methods() {
	
	ClassDB::bind_method(D_METHOD("add_animation_library", "name", "library"), &AnimationMixer::add_animation_library);
	ClassDB::bind_method(D_METHOD("remove_animation_library", "name"), &AnimationMixer::remove_animation_library);
	ClassDB::bind_method(D_METHOD("rename_animation_library", "name", "newname"), &AnimationMixer::rename_animation_library);