### 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
root_node = SceneStringNames::get_singleton()->path_pp;
}
AnimationMixer::~AnimationMixer() {
}
void AnimatedValuesBackup::set_data(const HashMap<NodePath, AnimationMixer::TrackCache *> p_data) {
	clear_data();
	for (const KeyValue<NodePath, AnimationMixer::TrackCache *> &E : p_data) {
		data.insert(E.key, get_cache_copy(E.value));
	}
}
HashMap<NodePath, AnimationMixer::TrackCache *> AnimatedValuesBackup::get_data() const {
	HashMap<NodePath, AnimationMixer::TrackCache *> ret;
	for (const KeyValue<NodePath, AnimationMixer::TrackCache *> &E : data) {
		ret.insert(E.key, get_cache_copy(E.value));
	}
	return ret;
}
void AnimatedValuesBackup::clear_data() {
	for (KeyValue<NodePath, AnimationMixer::TrackCache *> &K : data) {
		memdelete(K.value);
	}
	data.clear();
}
AnimationMixer::TrackCache *AnimatedValuesBackup::get_cache_copy(AnimationMixer::TrackCache *p_cache) const {
	switch (p_cache->type) {
		case Animation::TYPE_VALUE: {
			AnimationMixer::TrackCacheValue *src = static_cast<AnimationMixer::TrackCacheValue *>(p_cache);
			AnimationMixer::TrackCacheValue *tc = memnew(AnimationMixer::TrackCacheValue(*src));
			return tc;
		}