Adds Empty in the context of loops

Refs #2721, partially fulfilling it.

doc/Language/control.pod6

@@ -516,6 +516,14 @@ assign them to an array:
     my @a = do for 1, 2, 3 { $_ * 2 }; @a.say; # OUTPUT «[2 4 6]␤»
     my @b = (for 1, 2, 3 { $_ * 2 }); @b.say;  # OUTPUT: «[2 4 6]␤»
 
+The C<Empty> constant will act as a no-op for a loop:
+
+=for code
+say "Not here" for Empty;
+
+Will not do anything. This constant is
+L<equivalent to a empty Slip or List|/syntax/Empty>.
+
 =head1 X<gather/take|control flow,gather take>
 
 X<|lazy list,gather>X<|lazy list,take>