Fix EZP-23486: don't fallback to legacy on front-end preview

JIRA: https://jira.ez.no/browse/EZP-23486

Content preview in the frontend (using ezwt) does not use the `site_access` named parameter, hence no route was being matched.
This PR fixes it by adding an additional route, and making sure the `previewContentAction()` works as expected with a `null` `$siteAccessName`

Tests: Manual