plugins/pay: add comment on why we don't use an empty string

plugins/pay.c:879:7: error: zero-length gnu_printf format string [-Werror=format-zero-length]

Reported-by: @cdecker
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

plugins/pay.c

@@ -876,6 +876,7 @@ static struct command_result *handle_pay(struct command *cmd,
 
 	/* Get capacities of local channels. */
 	return send_outreq(cmd, "listpeers", listpeers_done, forward_error, pc,
+			   /* gcc doesn't like zero-length format strings! */
 			   " ");
 }