Add a simple selection rule for checking if a field is empty

Example use: making sure that group_audience field from OG is not empty before creating a relationship based on it.
Another example (assuming link tokens work): making sure a link field is not empty before using it as the path in a 301 redirection

This doesn't require anything that isn't in core, so it seems like it would be appropriate to have in ctools proper.

I'll be attaching a proposed patch in a moment.