There isn't any smart-collection criterion. However, you can find all photos without detected faces with a simple recipe:
1. In Library, choose All Photographs.
2. Do Photo > Stacking > Collapse All Stacks if you want to search just the top potos in stacks, or Expand All Stacks if you want to search all photos.
3. Do View > People.
4. Do Edit > Select All. Now, all photos with detected faces are selected.
5. Do View > Grid.
6. Do Edit > Invert Selection.
Now all photos that don't have detected faces are selected. You can add these to the Quick Collection or another collection to save them away.
Thanks John. That's very helpful. Now I just need to figure out how to run the Face Detection on those photos with people in it again. The 'Find Faces Again' just on those... This whole face detection thing is not all that mature yet. There also needs to be some way to clean up wrongly "confirmed faces" and retrain the detection after this..