ffi/unsafe/obj: ignore NULL protocols

If the `#:protocols` clauses of a `define-objc-class` form includes
errors, but it simplifies the declaration of protocols that are
introduced in different versions of a framework, and it's effectively
more compatible with the implementation before dc0898f5ef.

pkgs/racket-doc/scribblings/foreign/objc.scrbl

@@ -160,7 +160,8 @@ FFI type @racket[_Class]). The @racket[superclass-expr] should produce
 an Objective-C class or @racket[#f] for the superclass. An optional
 @racket[#:mixins] clause can specify mixins defined with
 @racket[define-objc-mixin]. An optional @racket[#:protocols] clause
-can specify Objective-C protocols to be implemented by the class.
+can specify Objective-C protocols to be implemented by the class, where
+a @racket[#f] result for a @racket[protocol-expr] is ignored.
 
 Each @racket[field-id] is an instance field that holds a Racket value
 and that is initialized to @racket[#f] when the object is
@@ -203,7 +204,10 @@ space for each @racket[field-id] within the instance is deallocated.
    (- _void (dealloc)
       (when bm (done-with-bm bm))))
  (void))
-]}
+]
+
+@history[#:changed "6.90.0.26" @elem{Changed @racket[#:protocols] handling to
+                                     ignore @racket[#f] expression results.}]}
 
 @defform[(define-objc-mixin (class-id superclass-id)
            maybe-mixins

racket/collects/ffi/unsafe/objc.rkt

@@ -729,11 +729,10 @@
       (objc_addClass (cast id _Class _objc_class-pointer))))
 
 (define (add-protocol id proto)
-  (unless proto
-    (error 'add-protocol "NULL protocol"))
-  (if class_addProtocol
-      (class_addProtocol id proto)
-      (add-protocol-the-hard-way id proto)))
+  (when proto
+    (if class_addProtocol
+        (class_addProtocol id proto)
+        (add-protocol-the-hard-way id proto))))
 
 (define (object-get-class id)
   (if object_getClass