PHP warnings when field has no columns

This is a very minor issue that is probably only going to show up for a handful of people. If a d6 source field has no columns defined, the unserialize call in populateSourceFieldInfo() doesn't return an empty array. This results in a php warning on the foreach a few lines down. I'm not sure if this case is even one that anyone else would run into (I have some custom fields I'm dealing with that may be malformed). The attached patch just does a quick sanity check to get rid off these errors.