### 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
ClassDB::bind_method(D_METHOD("has_animation_library", "name"), &AnimationMixer::has_animation_library);
	ClassDB::bind_method(D_METHOD("get_animation_library", "name"), &AnimationMixer::get_animation_library);
	ClassDB::bind_method(D_METHOD("get_animation_library_list"), &AnimationMixer::_get_animation_library_list);
	ClassDB::bind_method(D_METHOD("has_animation", "name"), &AnimationMixer::has_animation);
	ClassDB::bind_method(D_METHOD("get_animation", "name"), &AnimationMixer::get_animation);
	ClassDB::bind_method(D_METHOD("get_animation_list"), &AnimationMixer::_get_animation_list);
	
	ClassDB::bind_method(D_METHOD("set_active", "active"), &AnimationMixer::set_active);
	ClassDB::bind_method(D_METHOD("is_active"), &AnimationMixer::is_active);
	ClassDB::bind_method(D_METHOD("set_deterministic", "deterministic"), &AnimationMixer::set_deterministic);
	ClassDB::bind_method(D_METHOD("is_deterministic"), &AnimationMixer::is_deterministic);
	ClassDB::bind_method(D_METHOD("set_root_node", "path"), &AnimationMixer::set_root_node);
	ClassDB::bind_method(D_METHOD("get_root_node"), &AnimationMixer::get_root_node);
	ClassDB::bind_method(D_METHOD("set_callback_mode_process", "mode"), &AnimationMixer::set_callback_mode_process);
	ClassDB::bind_method(D_METHOD("get_callback_mode_process"), &AnimationMixer::get_callback_mode_process);
	ClassDB::bind_method(D_METHOD("set_callback_mode_method", "mode"), &AnimationMixer::set_callback_mode_method);
	ClassDB::bind_method(D_METHOD("get_callback_mode_method"), &AnimationMixer::get_callback_mode_method);
	ClassDB::bind_method(D_METHOD("set_audio_max_polyphony", "max_polyphony"), &AnimationMixer::set_audio_max_polyphony);
	ClassDB::bind_method(D_METHOD("get_audio_max_polyphony"), &AnimationMixer::get_audio_max_polyphony);
	
	ClassDB::bind_method(D_METHOD("set_root_motion_track", "path"), &AnimationMixer::set_root_motion_track);
	ClassDB::bind_method(D_METHOD("get_root_motion_track"), &AnimationMixer::get_root_motion_track);
	ClassDB::bind_method(D_METHOD("get_root_motion_position"), &AnimationMixer::get_root_motion_position);
	ClassDB::bind_method(D_METHOD("get_root_motion_rotation"), &AnimationMixer::get_root_motion_rotation);
	ClassDB::bind_method(D_METHOD("get_root_motion_scale"), &AnimationMixer::get_root_motion_scale);
	ClassDB::bind_method(D_METHOD("get_root_motion_position_accumulator"), &AnimationMixer::get_root_motion_position_accumulator);
	ClassDB::bind_method(D_METHOD("get_root_motion_rotation_accumulator"), &AnimationMixer::get_root_motion_rotation_accumulator);
	ClassDB::bind_method(D_METHOD("get_root_motion_scale_accumulator"), &AnimationMixer::get_root_motion_scale_accumulator);
	
	ClassDB::bind_method(D_METHOD("clear_caches"), &AnimationMixer::clear_caches);