Fix error when jQuery is trying to do toLowerCase an empty array.

I updated the version of jQuery of an old app. This app have a plugin called jqGrid, and in the method "setCell" is possible pass an Array of properties (attributes) for set them in a cell. The code have a lot of this and the guy who implemented it time ago the code have a lot of empty arrays set. So when jquery-migrate try to do a toLowerCase() of an empty array it throws an error. I don't know if is the best fix but works for me.