Call `return` when short-circuiting due to empty src

I saw a strange error when I had a misnamed `src` property of my Gruntfile.js;
it stemmed from the `async` lib and was due to the fact that in
`tasks/stylus.js`, `n()` could be called multiple times in a single iteration.
 There's a check for empty `src` (`srcFiles.length === 0`), which
short-circuits and calls `n()`, but doesn't `return`. Then, later down in the
function, `n()` gets called again, confusing the `async` lib.