### 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
return root_node;
}
void AnimationMixer::set_deterministic(bool p_deterministic) {
	deterministic = p_deterministic;
	_clear_caches();
}
bool AnimationMixer::is_deterministic() const {
	return deterministic;
}
void AnimationMixer::set_callback_mode_process(AnimationCallbackModeProcess p_mode) {
	if (callback_mode_process == p_mode) {
		return;
	}
	bool was_active = is_active();
	if (was_active) {
		set_active(false);
	}
	callback_mode_process = p_mode;
	if (was_active) {
		set_active(true);
	}
#ifdef TOOLS_ENABLED
	emit_signal(SNAME("mixer_updated"));
#endif 
}
AnimationMixer::AnimationCallbackModeProcess AnimationMixer::get_callback_mode_process() const {
	return callback_mode_process;
}
void AnimationMixer::set_callback_mode_method(AnimationCallbackModeMethod p_mode) {