message: vcall dispatch return zero when self is [nullptr]
diff: @@ -76,6 +76,8 @@ namespace detail {
  void* ptr = (void*)self.entry(0);
  if (ptr)
  result = func(ptr, args...);
  + else
  + result = zero<Result>();
  } else {
  enoki::schedule(args...);
  auto [buckets, size] = self.vcall_();
  