Fix EZP-23234: provide a default (null) locationId for _ezpublishLocation route

JIRA: https://jira.ez.no/browse/EZP-23234

This makes it possible to preview content using the internal `'_ezpublishLocation'` route if there is no actual location (new content draft).
The problem comes from using the `url()` twig helper and the checks done in Symfony.

From my limited knowledge, having a `null` default for the route should pose no issues, comments welcome.