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.

TODO:
 - [ ] Accept any callable
Letting PHP tell the difference between content strings and callback strings bugs me, but you may be right.