Fix User::idFromName() ignoring cache for non-existent users.
This fixes a database time out issue where User::idFromName() is repeatedly called from Special:Import due to the user not existing. The response is cached as null, but isset() will return false on a null key. I discovered this issue when trying to import a large XML file through Special:Import. The users for the revisions in the pages did not exist on the destination wiki so hundreds of queries from User:idFromName() were performed and ignoring the self::$idCacheByName cache.