A creaky convent horror in desperate need of absolution, The Nun is the latest in Warner Brothers’ credibility-stretching attempts to hoover up audience good will for The Conjuring.  Over two central films the franchise has become an accidental financial juggernaut for WB, its relatively straightforward scares crafted with confident professionalism by director James Wan, who has an eye for tension, and boosted by the amiable chemistry of Patrick Wilson and Vera Farmiga as betrothed Ghostbusters Ed and Lorraine Warren. The Conjuring 2 was especially solid, distinguished by an 1980s red brick terrace atmosphere. Its weakest moments were when it went full…