Try lookup via WKD even if key server is "none"

If only the lookup of S/MIME certificates is requested and there are no
directory servers configured then an error message is shown. Otherwise,
the user will be shown the lookup dialog because for OpenPGP lookups we
always use WKD even if key server lookups were disabled.

(cherry picked from commit 8effa10b823933c5b19bbfd91bab922aed77feef)

• GnuPG-bug-id: T6868