Allow plugins to return nothing

The Field Boxes module currently returns a box to render without having actual content on some pages. It would be nice to allow it to return NULL or an empty array if it decides that it doesn't actually have anything to display. See the diff for details - basically a one liner that does display a box if the rendered box is empty

Or if you want to be fancy, define up a hasDisplay() method, but personally, I would just go with the !empty() approach.

While this is a feature request, it bypasses multiple errors if an empty value is returned.