I'm definitely against the idea of putting `NSNull` in there, I can't count the amount of crashes I've had over the years because of `NSNull` getting sneaked into my codebase via this kind of thing. Take Mantle for example, it doesn't set a NSNull for nil from the server.

From Objective Sugar's perspective, this brings `NSSet` `-map` in-line with `NSArray` `-map` which does the same sanity check. They both should have the same internal API.