### 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
int at_pos = -1;
	for (uint32_t i = 0; i < animation_libraries.size(); i++) {
		if (animation_libraries[i].name == p_name) {
			at_pos = i;
			break;
		}
	}
	ERR_FAIL_COND(at_pos == -1);
	animation_libraries[at_pos].library->disconnect(SNAME("animation_added"), callable_mp(this, &AnimationMixer::_animation_added));
	animation_libraries[at_pos].library->disconnect(SNAME("animation_removed"), callable_mp(this, &AnimationMixer::_animation_removed));
	animation_libraries[at_pos].library->disconnect(SNAME("animation_renamed"), callable_mp(this, &AnimationMixer::_animation_renamed));
	animation_libraries[at_pos].library->disconnect(SNAME("animation_changed"), callable_mp(this, &AnimationMixer::_animation_changed));
	animation_libraries.remove_at(at_pos);
	_animation_set_cache_update();
	notify_property_list_changed();
}
void AnimationMixer::rename_animation_library(const StringName &p_name, const StringName &p_new_name) {