### 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("advance", "delta"), &AnimationMixer::advance);
	GDVIRTUAL_BIND(_post_process_key_value, "animation", "track", "value", "object", "object_idx");
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "active"), "set_active", "is_active");
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "deterministic"), "set_deterministic", "is_deterministic");
	ClassDB::bind_method(D_METHOD("set_reset_on_save_enabled", "enabled"), &AnimationMixer::set_reset_on_save_enabled);
	ClassDB::bind_method(D_METHOD("is_reset_on_save_enabled"), &AnimationMixer::is_reset_on_save_enabled);
	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "reset_on_save", PROPERTY_HINT_NONE, ""), "set_reset_on_save_enabled", "is_reset_on_save_enabled");
	ADD_SIGNAL(MethodInfo("mixer_updated")); 
	ADD_PROPERTY(PropertyInfo(Variant::NODE_PATH, "root_node"), "set_root_node", "get_root_node");
	ADD_GROUP("Root Motion", "root_motion_");
	ADD_PROPERTY(PropertyInfo(Variant::NODE_PATH, "root_motion_track"), "set_root_motion_track", "get_root_motion_track");
	ADD_GROUP("Audio", "audio_");
	ADD_PROPERTY(PropertyInfo(Variant::INT, "audio_max_polyphony", PROPERTY_HINT_RANGE, "1,127,1"), "set_audio_max_polyphony", "get_audio_max_polyphony");
	ADD_GROUP("Callback Mode", "callback_mode_");
	ADD_PROPERTY(PropertyInfo(Variant::INT, "callback_mode_process", PROPERTY_HINT_ENUM, "Physics,Idle,Manual"), "set_callback_mode_process", "get_callback_mode_process");
	ADD_PROPERTY(PropertyInfo(Variant::INT, "callback_mode_method", PROPERTY_HINT_ENUM, "Deferred,Immediate"), "set_callback_mode_method", "get_callback_mode_method");
	BIND_ENUM_CONSTANT(ANIMATION_CALLBACK_MODE_PROCESS_PHYSICS);
	BIND_ENUM_CONSTANT(ANIMATION_CALLBACK_MODE_PROCESS_IDLE);
	BIND_ENUM_CONSTANT(ANIMATION_CALLBACK_MODE_PROCESS_MANUAL);
	BIND_ENUM_CONSTANT(ANIMATION_CALLBACK_MODE_METHOD_DEFERRED);
	BIND_ENUM_CONSTANT(ANIMATION_CALLBACK_MODE_METHOD_IMMEDIATE);
	ADD_SIGNAL(MethodInfo(SNAME("animation_list_changed")));
	ADD_SIGNAL(MethodInfo(SNAME("animation_libraries_updated")));
	ADD_SIGNAL(MethodInfo(SNAME("animation_finished"), PropertyInfo(Variant::STRING_NAME, "anim_name")));
	ADD_SIGNAL(MethodInfo(SNAME("animation_started"), PropertyInfo(Variant::STRING_NAME, "anim_name")));
	ADD_SIGNAL(MethodInfo(SNAME("caches_cleared")));
	ClassDB::bind_method(D_METHOD("_reset"), &AnimationMixer::reset);
	ClassDB::bind_method(D_METHOD("_restore", "backup"), &AnimationMixer::restore);
}
AnimationMixer::AnimationMixer() {