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.
Merge pull request #73 from spikebrehm/fix/return-on-empty-src

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