andthen returns Empty, not first undefined arg

By design, to make it flatten into lists

Author: zoffixznet

doc/Language/operators.pod6

@@ -2256,7 +2256,8 @@ to misuse, see L<traps|/language/traps#Loose_boolean_operators>.
 
 =head2 infix C«andthen»
 
-Returns the first undefined argument, otherwise the last argument.
+Returns L«C<Empty>|/type/Slip#index-entry-Empty-Empty» unpon encountering
+the first undefined argument, otherwise the last argument.
 Short-circuits.  The result of the left side is bound to $_ for the
 right side, or passed as arguments if the right side is a block or
 pointy block.