pmb.config.init: Improve message when "Archived:" header is missing
This changes the output from e.g.
WARNING: xiaomi-markw is archived: None
to
WARNING: xiaomi-markw is archived: No reason given (this is a bug)
which I think is better and makes it clear that this is not behaviour one should rely upon. All archived ports should have a reason given, but this has not been (and still isn't as I'm writing this) enforced in pmaports, so we can't rely on there being one.
Related: pmaports!6085 (merged)