### 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
to_delete.push_back(K.key);
		}
	}
	while (to_delete.front()) {
		NodePath np = to_delete.front()->get();
		memdelete(track_cache[np]);
		track_cache.erase(np);
		to_delete.pop_front();
	}
	track_map.clear();
	int idx = 0;
	for (const KeyValue<NodePath, TrackCache *> &K : track_cache) {
		track_map[K.key] = idx;
		idx++;
	}
	track_count = idx;
	cache_valid = true;
	return true;
}
void AnimationMixer::_process_animation(double p_delta, bool p_update_only) {
	_blend_init();
	if (_blend_pre_process(p_delta, track_count, track_map)) {
		_blend_calc_total_weight();
		_blend_process(p_delta, p_update_only);
		_blend_apply();
		_blend_post_process();
	};
	clear_animation_instances();
}
Variant AnimationMixer::post_process_key_value(const Ref<Animation> &p_anim, int p_track, Variant p_value, const Object *p_object, int p_object_idx) {