Tack on a (nil) argument if the selector is asking for one

Seems like pre-ARC would tack on a `nil` argument when calling `performSelector:` on a selector that required an argument, but for which none was provided.
Now, we have to explicitly check if the selector requires an argument, and tack one on (`nil` in this case) if none is provided.

Fixes @philostein's bug found Rob [mentioned](https://github.com/quicksilver/Quicksilver/issues/1716#issuecomment-30711545)