racket/class: fix empty define-values

Closes PR 14451

pkgs/racket-pkgs/racket-test/tests/racket/object.rktl

@@ -281,6 +281,8 @@
 (class-keyword-test #'super-instantiate)
 (class-keyword-test #'inspect)
 
+(test #t class? (class object% (define-values () (values))))
+
 ;; ------------------------------------------------------------
 ;; Test basic functionality
 

racket/collects/racket/private/class-internal.rkt

@@ -1207,7 +1207,8 @@
                                                          (syntax/loc e
                                                            (let-values ([(temp ...) expr])
                                                              (set! id (field-initialization-value temp))
-                                                             ...))))])
+                                                             ...
+                                                             (void)))))])
                                              (syntax-track-origin assignment e #'d-v))]
                                           [(_init orig idp ...)
                                            (and (identifier? (syntax _init))