title: Do something when slot does not have child nodes in lit-html
body: <p>I'd like to display a text when no content is on the slot.</p>
  
  <pre class="lang-js prettyprint-override"><code>class List extends LitElement {
    public render() {
      return slot.length === 0 
        ? html`No content is available`
        : html`&lt;slot&gt;&lt;/slot&gt;`;
    }
  }
  </code></pre>