### 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
case Animation::TYPE_POSITION_3D:
		case Animation::TYPE_ROTATION_3D:
		case Animation::TYPE_SCALE_3D: {
			AnimationMixer::TrackCacheTransform *src = static_cast<AnimationMixer::TrackCacheTransform *>(p_cache);
			AnimationMixer::TrackCacheTransform *tc = memnew(AnimationMixer::TrackCacheTransform(*src));
			return tc;
		}
		case Animation::TYPE_BLEND_SHAPE: {
			AnimationMixer::TrackCacheBlendShape *src = static_cast<AnimationMixer::TrackCacheBlendShape *>(p_cache);
			AnimationMixer::TrackCacheBlendShape *tc = memnew(AnimationMixer::TrackCacheBlendShape(*src));
			return tc;
		}
		case Animation::TYPE_BEZIER: {
			AnimationMixer::TrackCacheBezier *src = static_cast<AnimationMixer::TrackCacheBezier *>(p_cache);
			AnimationMixer::TrackCacheBezier *tc = memnew(AnimationMixer::TrackCacheBezier(*src));
			return tc;
		}
		case Animation::TYPE_AUDIO: {
			AnimationMixer::TrackCacheAudio *src = static_cast<AnimationMixer::TrackCacheAudio *>(p_cache);
			AnimationMixer::TrackCacheAudio *tc = memnew(AnimationMixer::TrackCacheAudio(*src));
			return tc;
		}
		case Animation::TYPE_METHOD:
		case Animation::TYPE_ANIMATION: {
			
		} break;
	}
	return nullptr;
}