You might want to return NSNull instead of nil (easy to do: in fact I extracted this from an object in which the nil-return value is configurable but simplified it for the post), in which case it becomes map-safe because NSNull->NSNull on map.

I use this pattern because then I can trust _everything_ in a collection. If an object, even an object that represents emptiness, receives a message, it will handle it and do something, even where that something represents emptiness.