I think I'd prefer number 2 over yet another NSNull-nil-map.
So you agree to not making any assumptions what `nil` means in `-transformedValue:` and making `-mtl_transformedValue:error:` opt-in?

![disappointment](https://f.cloud.github.com/assets/212465/1270815/e3d85d10-2d0f-11e3-8eb3-c3c226eed4c7.gif)