Do not load special pages in WatchedItem::load

Backporting https://github.com/wikimedia/mediawiki-core/commit/607449cb84e20c5b1035f4b6867b3d1ac6b26567#diff-64fe2609dd41ca7d16d8dff717036ed9R32

WatchedItem::load was looking for a special page in the database:

SELECT  wl_notificationtimestamp
FROM `watchlist`
WHERE wl_user = '1'
AND wl_namespace = '-1'
AND wl_title = 'Log'
LIMIT 1

But it is known that such a page never will stay in the database,
so added a check to avoid the select.