Fixed EZP-22823: exception in Preview w/ website toolbar

> Fixes http://jira.ez.no/browse/EZP-22823

An exception is thrown by Twig/Stash if the websitetoolbar is enabled with the provided configuration, and a Preview is requested.

In the PreviewController, a fake Location is created, with among others `id`set to `null`. This null ID is passed on to the website toolbar render, and content is loaded by the API from a null ID.

If we go this way, it also needs to be documented like this.