### 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
const Variant **argptrs = (const Variant **)alloca(sizeof(Variant *) * p_params.size());
	const Variant *args = p_params.ptr();
	uint32_t argcount = p_params.size();
	for (uint32_t i = 0; i < argcount; i++) {
		argptrs[i] = &args[i];
	}
	if (p_deferred) {
		MessageQueue::get_singleton()->push_callp(p_object, p_method, argptrs, argcount);
	} else {
		Callable::CallError ce;
		p_object->callp(p_method, argptrs, argcount, ce);
	}
}
void AnimationMixer::make_animation_instance(const StringName &p_name, const PlaybackInfo p_playback_info) {
	ERR_FAIL_COND(!has_animation(p_name));
	AnimationData ad;
	ad.name = p_name;
	ad.animation = get_animation(p_name);
	ad.animation_library = find_animation_library(ad.animation);
	AnimationInstance ai;
	ai.animation_data = ad;
	ai.playback_info = p_playback_info;
	animation_instances.push_back(ai);
}
void AnimationMixer::clear_animation_instances() {
	animation_instances.clear();
}
void AnimationMixer::advance(double p_time) {