### 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
for (const AnimationLibraryData &lib : animation_libraries) {
		p_libraries->push_back(lib.name);
	}
}
Ref<AnimationLibrary> AnimationMixer::get_animation_library(const StringName &p_name) const {
	for (const AnimationLibraryData &lib : animation_libraries) {
		if (lib.name == p_name) {
			return lib.library;
		}
	}
	ERR_FAIL_V(Ref<AnimationLibrary>());
}
bool AnimationMixer::has_animation_library(const StringName &p_name) const {
	for (const AnimationLibraryData &lib : animation_libraries) {
		if (lib.name == p_name) {
			return true;
		}
	}
	return false;
}
StringName AnimationMixer::find_animation_library(const Ref<Animation> &p_animation) const {
	for (const KeyValue<StringName, AnimationData> &E : animation_set) {
		if (E.value.animation == p_animation) {
			return E.value.animation_library;
		}
	}
	return StringName();
}
Error AnimationMixer::add_animation_library(const StringName &p_name, const Ref<AnimationLibrary> &p_animation_library) {