divisors returns empty list for 0

This is mainly for consistency with other words
(factors, group-factors, unique-factors)

basis/math/primes/factors/factors.factor

@@ -47,8 +47,10 @@ PRIVATE>
     dup 1 = [
         1array
     ] [
-        group-factors [ first2 [0,b] [ ^ ] with map ] map
+        group-factors dup empty? [
+            [ first2 [0,b] [ ^ ] with map ] map
             [ product ] product-map natural-sort
+        ] unless
     ] if ;
 
 : unix-factor ( string -- )