Initialize fields if CodeBlobIterator shortcuts without heaps

SonarCloud reports that `CodeBlobIterator::_code_blob` is not initialized on shortcut path:

```
  CodeBlobIterator(LivenessFilter filter, T* nm = NULL)
    : _only_not_unloading(filter == only_not_unloading)
  {
    if (Filter::heaps() == NULL) {
      return; <---- here
    }
```

It seems that we don't have an actual bug here, as iterator shortcuts on `_heap == _end`, but it would be cleaner to avoid garbage anyway.