According to the html5 standard, there must be a title in the nav tag. However, the TOC is generated without the ability to specify this header

It should be like this. This helps visually impaired browsers to read what this section is responsible for.

<nav>
  <h1>Navigation</h1>
  <ol>
    <li></li>
    <li></li>
  </ol>
</nav>

P.S. If you wish, you can make sure that this is done in Google. They add an h tag for the sections while hiding it, since it is only needed for semantic markup.