fix object/c & fields that just have to exist

closes PR 14437

pkgs/racket-pkgs/racket-test/tests/racket/contract/object.rkt

@@ -158,4 +158,32 @@
                       'pos
                       'neg)])
     (set-field! n pre-o #t)
-    (get-field n o))))
+    (get-field n o)))
+
+(test/spec-passed/result
+ 'object/c-field-existence
+ '(send (contract 
+         (object/c
+          (field foo bar)
+          (baz (->m integer? integer?)))
+         (new (class object%
+                (super-new)
+                (field (foo 0) (bar 0))
+                (define/public (baz n) n)))
+         'pos 'neg)
+        baz 1)
+ 1)
+
+(test/spec-passed/result
+ 'object/c-field-existence2
+ '(send (contract 
+         (object/c
+          (field foo bar)
+          (baz (->m integer? (listof integer?))))
+         (new (class object%
+                (super-new)
+                (field (foo 0) (bar 1))
+                (define/public (baz n) (list foo bar))))
+         'pos 'neg)
+        baz 1)
+ '(0 1)))

racket/collects/racket/private/class-c-old.rkt

@@ -1307,7 +1307,7 @@
     (unless (null? fields)
       (for ([f (in-list fields)]
             [c (in-list field-contracts)])
-        (when c
+        (unless (just-check-existence? c)
           (define fi (hash-ref field-ht f))
           (define p-pos ((contract-projection c) (blame-add-field-context blame f #:swap? #f)))
           (define p-neg ((contract-projection c) (blame-add-field-context blame f #:swap? #t)))