cppgc: Fix gcc compilation of FinalizerTrait

finalizer-trait.h:79:49: error: the address of ‘static void cppgc::internal::FinalizerTrait<T>::Finalize(void*) [with T = Rope]’ will never be NULL [-Werror=address]
   static constexpr bool HasFinalizer() { return kCallback; }

Fixes: 
Change-Id: I368138e37189440d786f130a1bce3577b7c0220f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3097267
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76295}

include/cppgc/internal/finalizer-trait.h

@@ -76,7 +76,7 @@ struct FinalizerTrait {
   }
 
  public:
-  static constexpr bool HasFinalizer() { return kCallback; }
+  static constexpr bool HasFinalizer() { return kNonTrivialFinalizer; }
 
   // The callback used to finalize an object of type T.
   static constexpr FinalizationCallback kCallback =