docs: public static <T> List<CloseableReference<T>> cloneOrNull(
        @PropagatesNullable Collection<CloseableReference<T>> refs) {
      if (refs == null) {
        return null;
      }
      List<CloseableReference<T>> ret = new ArrayList<>(refs.size());
      for (CloseableReference<T> ref : refs) {
        ret.add(CloseableReference.cloneOrNull(ref));
      }
      return ret;
    }