### 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
#include "animation_mixer.h"
#include "core/config/engine.h"
#include "scene/animation/animation_player.h"
#include "scene/resources/animation.h"
#include "scene/scene_string_names.h"
#include "servers/audio/audio_stream.h"
#ifdef TOOLS_ENABLED
#include "editor/editor_node.h"
#include "editor/editor_undo_redo_manager.h"
#endif 
bool AnimationMixer::_set(const StringName &p_name, const Variant &p_value) {
	String name = p_name;
#ifndef DISABLE_DEPRECATED
	if (name.begins_with("anims/")) {
		
		String which = name.get_slicec('/', 1);
		Ref<Animation> anim = p_value;