如何在父组件中获取子组件 render 的返回值呢？

想在 content 之间插入一些内容，但是要怎么在父组件中判断子组件的渲染结果是不是 null 呢？

代码类似

https://codesandbox.io/s/k9nr4pqz9r

function LI({ show = true }) {
  if (show) {
    return <span>content</span>;
  }
  return null;
}

function UL({ children }) {
  let ret = [];
  React.Children.forEach(children, (child, index) => {
    if (true /* how to get child render result ? */) {
      ret.push(child);
      ret.push(" * ");
    }
  });
  if (ret.length) {
    ret.pop();
  }
  return ret;
}

function App() {
  return (
    <>
      <UL>
        <LI />
        <LI />
        <LI />
        <LI />
      </UL>
      <hr />
      <UL>
        <LI />
        <LI show={false} />
        <LI />
        <LI />
      </UL>
    </>
  );
}

so 链接： https://stackoverflow.com/questions/53201879/how-do-i-determine-in-the-parent-component-whether-the-rendering-result-of-the-c