code: static <F, T> Function<F, T> fallback(
        Function<F, T> function, Function<? super F, ? extends T> defaultFunction) {
      return from -> {
        T result = function.apply(from);
        return result == null ? defaultFunction.apply(from) : result;
      };
    }
docs: Returns a {@link Function} that delegates to {@code function} and falls back to
  {@code defaultFunction} for null return values.