Missing 404.html and sitemap.xml because the demo playbook does not specify a URL

Trying out the demo project playbook (https://gitlab.com/antora/demo/demo-site/blob/master/site.yml) for getting started docs of antora, I got puzzled by not finding the 404.html and sitemap as expected from reading https://docs.antora.org/antora/1.0/run-antora/

Wandering around the code source I found out that 404 is not generated because the demo playbook does not specify a URL https://gitlab.com/antora/antora/blob/master/packages/site-generator-default/lib/generate-site.js#L27

I suppose because the generated 404 contains the URL in the header

<a class="navbar-item" href="http://localhost:5000">Antora Demo Site</a>
```
but all the other links in this 404 page are relative such as ` <a href="/component-b/2.0/index.html">2.0</a>`

so the first link could have been written href="/" anyway without depending on the URL, no ?

First though would be to make the site.url parameter mandatory ? 
or could the demo playbook have a default value `url: /` ?