use unsafe-undefined for missing arguments in string-join

This is similar to the recent change of functions with optional
values. Using unsafe-undefined instead of a gensym makes it easier
to avoid the check of the missing argument.

racket/collects/racket/string.rkt

@@ -19,9 +19,8 @@
                [(s1 s2 s3 s4 strs) (apply string-append s1 s2 s3 s4 strs)]
                [(str . strss) (apply apply string-append str strss)]))
 
-(require (only-in racket/list add-between))
-
-(define none (gensym))
+(require (only-in racket/list add-between)
+         (only-in racket/unsafe/undefined [unsafe-undefined none]))
 
 (define (string-join strs [sep " "]
                      #:before-first [before-first none]