Conditionally rendering nothing on an aria attribute produces an error when it shouldn't

[mark-dropbear]

The code below is an example taken from an element class that demonstrates a scenario that triggers the following error on the aria-current attribute for the link element.

Type '"page" | Symbol(undefined)' is not assignable to '"page" | "step" | "location" | "date" | "time" | "true" | "false"'lit-plugin(no-incompatible-type-binding)(2304)

#renderLinks(): TemplateResult {
  return html`
    <ul>
      ${this.#menuItems.map((link) => {
      return html`
        <li>
          <a href="${link.href.pathname}" aria-current="${this.#isCurrentPage(link.href) || nothing }">${link.text}</a>
        </li>
      `})
    }
    </ul>
  `;
}

#isCurrentPage(url: URL){
  if(url.pathname === window.location.pathname){
    return 'page';
  } else {
    return undefined;
  }
}

This should not trigger an error as per these guidelines: https://lit.dev/docs/templates/conditionals/#conditionally-rendering-nothing

[Jeff Dairiki]

Possible dup of #251