Revision::newNullRevision: Make sure the Title is set in the constructor

Revision::newNullRevision: Make sure the Title is set in the constructor

Revision::__construct() calls Revision::getContentModel() to initialize
the 'content_model' field. If a Title object is not set, it will try to
look up the page in the slave database, which might be behind. At that
point it will default to wikitext, which is problematic during imports.

By passing the Title object in the constructor instead of afterwards,
the content model should be intialized properly.

Bug: T91170
Change-Id: I7fea996ea5b723967272af8cb344150aea10bb54