avoid arithmetic exception

Author: Johannes Waldmann

containers/src/Data/Set/Internal.hs

@@ -1828,6 +1828,7 @@ splitRoot orig =
 -- @since 0.5.11
 
 powerSet :: Set a -> Set (Set a)
+powerSet xs | null xs = singleton xs
 powerSet xs =
   let !w = length xs
       !u = A.listArray (0, w-1) $ toList xs