### 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 (p_name == p_new_name) {
		return;
	}
#ifdef DEBUG_ENABLED
	ERR_FAIL_COND_MSG(String(p_new_name).contains("/") || String(p_new_name).contains(":") || String(p_new_name).contains(",") || String(p_new_name).contains("["), "Invalid animation library name: " + String(p_new_name) + ".");
#endif
	bool found = false;
	for (AnimationLibraryData &lib : animation_libraries) {
		ERR_FAIL_COND_MSG(lib.name == p_new_name, "Can't rename animation library to another existing name: " + String(p_new_name) + ".");
		if (lib.name == p_name) {
			found = true;
			lib.name = p_new_name;
			
			lib.library->disconnect(SNAME("animation_added"), callable_mp(this, &AnimationMixer::_animation_added));
			lib.library->disconnect(SNAME("animation_removed"), callable_mp(this, &AnimationMixer::_animation_removed));
			lib.library->disconnect(SNAME("animation_renamed"), callable_mp(this, &AnimationMixer::_animation_renamed));
			lib.library->connect(SNAME("animation_added"), callable_mp(this, &AnimationMixer::_animation_added).bind(p_new_name));
			lib.library->connect(SNAME("animation_removed"), callable_mp(this, &AnimationMixer::_animation_removed).bind(p_new_name));
			lib.library->connect(SNAME("animation_renamed"), callable_mp(this, &AnimationMixer::_animation_renamed).bind(p_new_name));
			for (const KeyValue<StringName, Ref<Animation>> &K : lib.library->animations) {
				StringName old_name = p_name == StringName() ? K.key : StringName(String(p_name) + "/" + String(K.key));
				StringName new_name = p_new_name == StringName() ? K.key : StringName(String(p_new_name) + "/" + String(K.key));
				_rename_animation(old_name, new_name);
			}
		}
	}
	ERR_FAIL_COND(!found);
	animation_libraries.sort();