feature(views): allows rendering empty results using an anonymous function

If you want the no_results message to not be wrapped by P tags, or you don’t
want to always incur the cost of rendering a view for backup content, you
can now provide a Closure whose output will be echoed by the view.
Merge pull request #7349 from mrclay/no_results_closure

feature(views): allows rendering empty results using an anonymous function