code: 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;
    }
docs: Clones a collection of references and returns a list. Returns null if the list is null. If the
  list is non-null, clones each reference. If a reference cannot be cloned due to already being
  closed, the list will contain a null value in its place.
  
  @param refs the references to clone
  @return the list of cloned references or null