Add support for memoizing nil

[npostavs]

;; There's no way to memoize nil returns, but why would your expensive ;; functions do all that work just to return nil? :-)

I had an example of an expensive function returning nil: http://emacs.stackexchange.com/a/19551/5296 (although it additionally needs memoizing only some arguments) and it's not so hard to memoize it, so why not?

[alphapapa]

FYI, the function added in https://github.com/skeeto/emacs-memoize/pull/10 handles this.