Card: Remove the unnecessary "reality check" binding on footer, add test
HeaderFooterLayout, just like QQC2.Page and generally all the Controls, takes care of orphaning an old header/contentItem/footer items, which effectively makes them invisible.
Ideally, it should be using deferred execution, so that an overridden item is deleted right away or not created in the first place. But that's a private API ¯\_(ツ)_/¯