This isn't necessary, the value sent from `signalForSelector:` doesn't need to be mapped to `nil`.

A couple notes:

* Despite clang letting it happen, `-reduceEach:` takes a block of one or more parameters. The number of parameters must match the number of values in the tuples sent in the signal it's applied to. In this case, the tuple will have just one value, so `-reduceEach:` needs a block with one parameter.
* Whenever you're returning a constant value from `-map:` or `-reduceEach:`, you can instead of use `-mapReplace:`.