Avoid select/writes for anon user in WatchedItem

For a page view of a anon user the following select is executed:
SELECT  wl_notificationtimestamp  FROM `watchlist`  WHERE wl_user = '0'
AND wl_namespace = '0' AND wl_title = 'Hauptseite'  LIMIT 1  

Change-Id: I1cf77a2d024956ed7c150bdf1687eb80c7eccc6f