Early return when Grunticon has no files to read

If no files are passed to Grunticon, it will fail with the error
"Grunticon has no files to read!", and call the async `done()` callback.
However, it will continue to execute the task, which will fail when
Grunticon tries to access the first file.

This pull request adds an early return after the `done()` callback is
called when Grunticon has no files to read. This will prevent execution
after the warning message is displayed, which will make the error more
obvious, avoid crashing, and make debugging easier.