To find images with no keywords at all, do the following:
In contact sheet view, click on the "Sort by:" pull-down menu.
Select "Edit Custom..."
Under "Primary" enter "{keywords}"
(Optional: If you wish, you can change the name of the search to reflect this change, or you can save it as a snapshot by clicking on the lightning bolt.)
Click OK
Now your images will be sorted according to their keywords. Images without any keywords will appear first.
Kevin