XWIKI-9847: Distribution Wizard first step does not see empty databases anymore
* Improve the test by counting only the documents that are not XClasses. There are still 3 documents like this that are automatically created (Watchlist notifiers) but the condition should be more robust now.

(cherry picked from commit c7f3a8673fae4700b79b1217cce750e2c12a61d8)