add support for empty element references to expr_build_next

template <typename... T>

ExprStorage expr_build_next(ExprStorage head, T&&... tail) {

  auto e = std::move(head);

  auto n = expr_build_next(std::forward<T>(tail)...);

  if (!head) {

    return n;

  }

  auto e = std::move(head);

  expr_set_next(e.get(), std::move(n));

  return e;

}