Fix default fallback for contract-struct-generate

The actual problem is that build-chaperone-contract-property
exported to the user defaults #:exercise and #:generate
to false. This commit changes the default fallback value
in the case where #:generate is not a procedure instead
of changing build-chaperone-contract-property directly
to stay consistent with the how contract-struct-exercise
currently does it.

pkgs/racket-test/tests/racket/contract/random-generate.rkt

@@ -435,12 +435,20 @@
      #:late-neg-projection
      (λ (ctc) (λ (b) (λ (v np) v)))))
   (define impersonate-any/c (impersonate-any/c-struct))
+  (struct chaperone-proc?/c-struct ()
+    #:property prop:chaperone-contract
+    (build-chaperone-contract-property
+     #:late-neg-projection
+     (λ (ctc) (λ (b) (λ (v np) v)))))
+  (define chaperone-proc?/c (chaperone-proc?/c-struct))
   (check-exn cannot-generate-exn? (λ () (test-contract-generation impersonate-any/c)))
   (check-exn cannot-generate-exn?
              (λ ()
                (test-contract-generation
                 (->i ([n integer?])
-                     [_ (n) (λ (r) (eq? r (even? n)))])))))
+                     [_ (n) (λ (r) (eq? r (even? n)))]))))
+  ;; Testing the default return value for contract-struct-generate
+  (check-exn cannot-generate-exn? (λ () (test-contract-generation chaperone-proc?/c))))
 
 (check-exercise
  10

racket/collects/racket/contract/private/prop.rkt

@@ -199,7 +199,7 @@
   (define generate (contract-property-generate prop))
   (if (procedure? generate)
       (generate c)
-      #f))
+      (λ (fuel) #f)))
 
 (define (contract-struct-exercise c)
   (define prop (contract-struct-property c))