### 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 (processing == p_process && !p_force) {
		return;
	}
	switch (callback_mode_process) {
		case ANIMATION_CALLBACK_MODE_PROCESS_PHYSICS:
#ifdef TOOLS_ENABLED
			set_physics_process_internal(p_process && active && !editing);
#else
			set_physics_process_internal(p_process && active);
#endif 
			break;
		case ANIMATION_CALLBACK_MODE_PROCESS_IDLE:
#ifdef TOOLS_ENABLED
			set_process_internal(p_process && active && !editing);
#else
			set_process_internal(p_process && active);
#endif 
			break;
		case ANIMATION_CALLBACK_MODE_PROCESS_MANUAL:
			break;
	}
	processing = p_process;
}
void AnimationMixer::set_active(bool p_active) {