New posts pass PostsController sorting function at the top

Following on #3008, it seems the PostsController tries to sort the newly-created post model when the user goes to the `edit.new` route or saves there, when starting out from the main admin screen. This model doesn't have `updated_at` or `published_at` dates, and so will not otherwise pass the sorting function.
I'm with it. I'll update with that change.