Currently, it appears that zero-dimension shapes used in shape-outside and clip-path are deemed invalid and not used. Try a rectangle(0,0,0px,0px) as either a shape-outside or clip-path to see that the float area and clip area remain unchanged.
The spec says that negative lengths are invalid, which means that zero should be OK. It should be useful to be able to use a zero-extent clip area in an irising transition.
As far as I can tell, this has already been fixed. rectangle() is slated for removal, but your example works just fine in the latest nightly. I also tried: circle(0), ellipse(0), and polygon(0 0), all of which properly gave a zero extent shape.