only call reporter if there is a message to report

I am using gulp-notify to give notifications from gulp-jshint, which will add information about a run to the file object. With the `notify(Function)` syntax, I am able to build up a short message and display it, which works very well.

The problem is that when there is no errors to report, a notification still pops up. I can't really think of a use case where you would want to report nothing, so this basically just adds a check to see if there is a message before invoking the reporter. 

If there is a use case I am not thinking of, I still think that returning undefined from the function should indicate that you don't want the reporter to run. If you like, I can modify the PR to only do that.