XWiki Navigation tree with no child have an expand button by default

For XWiki, fixing this TODO should be enough. The issue exists with the other backends as well and will probably require checking manually if children exist (which is not hard to do through `NavigationTreeSource.getChildNodes()`, but possibly expensive).

The alignment issue is mostly due to the lack of CSS customization for the Vuetify ds (this is the expected look for a node without children right now), which will hopefully be covered by https://jira.xwiki.org/browse/CRISTAL-231