### 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
}
void AnimationMixer::_blend_calc_total_weight() {
	for (const AnimationInstance &ai : animation_instances) {
		Ref<Animation> a = ai.animation_data.animation;
		real_t weight = ai.playback_info.weight;
		Vector<real_t> track_weights = ai.playback_info.track_weights;
		Vector<int> processed_indices;
		for (int i = 0; i < a->get_track_count(); i++) {
			if (!a->track_is_enabled(i)) {
				continue;
			}
			NodePath path = a->track_get_path(i);
			if (!track_cache.has(path)) {
				continue; 
			}
			TrackCache *track = track_cache[path];
			int blend_idx = track_map[path];
			if (processed_indices.has(blend_idx)) {
				continue; 
			}
			ERR_CONTINUE(blend_idx < 0 || blend_idx >= track_count);
			real_t blend = blend_idx < track_weights.size() ? track_weights[blend_idx] * weight : weight;
			track->total_weight += blend;
			processed_indices.push_back(blend_idx);
		}
	}
}
void AnimationMixer::_blend_process(double p_delta, bool p_update_only) {