Restore logging if no completion handler given

The logging in `NSManagedObjectContext+DCTCoreDataStack.m` was never called because the override in `_DCTCDSManagedObjectContext` was adding a no-op completion handler if none was passed in.

Moving the logging to the completion handler added by `_DCTCDSManagedObjectContext` fixes this.

(Fix suggested by Daniel Tull)