BUG: Tethered site children without uriPathSegment can never get one

[nezaniel]

Is there an existing issue for this?

• I have searched the existing issues

Current Behavior

When tethered child documents are added to a site node without a default value for uriPathSegment, they will not get their own URI

Expected Behavior

uriPathSegments that are added later should also take effect

Steps To Reproduce

• add a tethered document to your site node type without default uriPathSegment
• run a creation command or structureAdjustments:fix
  -> the tethered child will show up instead of the homepage on path '/'

Environment

- Flow: 9.0-dev
- Neos: 9.0-dev
- PHP: 8.0

Anything else?

This happens because in DocumentUriPathProjection::550 and following:

$oldUriPath = $node->getUriPath();
// homepage -> TODO hacky?
if ($oldUriPath === '') {
  continue;
}

Instead of checking for empty string, we should check against the site node type introduced in #4563