### 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
} break;
			case Animation::TYPE_BLEND_SHAPE: {
				TrackCacheBlendShape *t = static_cast<TrackCacheBlendShape *>(track);
				t->value = t->init_value;
			} break;
			case Animation::TYPE_VALUE: {
				TrackCacheValue *t = static_cast<TrackCacheValue *>(track);
				t->value = Animation::cast_to_blendwise(t->init_value);
				t->element_size = t->init_value.is_string() ? (real_t)(t->init_value.operator String()).length() : 0;
			} break;
			case Animation::TYPE_BEZIER: {
				TrackCacheBezier *t = static_cast<TrackCacheBezier *>(track);
				t->value = t->init_value;
			} break;
			case Animation::TYPE_AUDIO: {
				TrackCacheAudio *t = static_cast<TrackCacheAudio *>(track);
				for (KeyValue<ObjectID, PlayingAudioTrackInfo> &L : t->playing_streams) {
					PlayingAudioTrackInfo &track_info = L.value;
					track_info.volume = 0.0;
				}
			} break;
			default: {
			} break;
		}
	}
}
bool AnimationMixer::_blend_pre_process(double p_delta, int p_track_count, const HashMap<NodePath, int> &p_track_map) {
	return true;
}
void AnimationMixer::_blend_post_process() {