nil
if the array is empty.
*
- * a = [ "a", "m", "z" ]
- * a.pop #=> "z"
- * a #=> ["a", "m"]
+ * a = [ "a", "b", "c", "d" ]
+ * a.pop #=> "d"
+ * a.pop(2) #=> ["b", "c"]
+ * a #=> ["a"]
*/
static VALUE