Fix error in ToC script at the bottom of the page

[zecakeh]

It didn't account for the case where there is no visible heading at the bottom of the page.

The error should be easy to reproduce in the current unstable spec, by navigating to the very bottom of the CS spec. A bunch or errors should appear in the console log.

Pull Request Checklist

• Pull request includes a changelog file
• Pull request includes a sign off
• Pull request is classified as 'other changes'

Preview: https://pr2002--matrix-spec-previews.netlify.app

[richvdh]

Indeed. Error is:

22:04:08.829 Uncaught TypeError: heading is null
    onScroll https://spec.matrix.org/unstable/js/toc.js:155
    throttle https://spec.matrix.org/unstable/js/toc.js:39
    EventListener.handleEvent* https://spec.matrix.org/unstable/js/toc.js:162
    EventListener.handleEvent* https://spec.matrix.org/unstable/js/toc.js:147
toc.js:155:20

This was presumably introduced by #1991.

[richvdh]

Thanks!