The new change form was using `default_if_none` to specify the defaults for visibility of various buttons, but that doesn't do the right thing if the variables being checked just aren't defined at all in the context. This change fixes it to use `default` instead, and enables visibility of the "Save and add another" and "Save and continue editing" by default. Testing Done: Opened the admin form for creating new objects (repositories, users, groups) and saw that the submit row buttons were now visible.