Refactored sender address evaluation.

See https://invent.kde.org/pim/kleopatra/-/merge_requests/16#note_427795
and https://invent.kde.org/pim/kleopatra/-/merge_requests/16#note_427796

Since a restriction is in place which allows for only one key to be
published using WKS, it is also unnecessary to iterate over d->keys()
as it is guaranteed there's only one key in that vector.