### 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
if (active == p_active) {
		return;
	}
	active = p_active;
	_set_active(active);
	_set_process(processing, true);
	if (!active && is_inside_tree()) {
		_clear_caches();
	}
}
bool AnimationMixer::is_active() const {
	return active;
}
void AnimationMixer::set_root_node(const NodePath &p_path) {
	root_node = p_path;
	_clear_caches();
}
NodePath AnimationMixer::get_root_node() const {