Don't draw anything in TableView unless there is a column

I was really confused because after assigning a model it looked like
the table view was showing the data, while in reality it was
just filling up with alternating row colors. I think it is
better not to draw anything to ensure that people correctly add
a TableViewColumn.

Change-Id: I353b478f4af80410aca3f94439a02ed7ab53f6d6
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Caroline Chao <caroline.chao@digia.com>