sm: Emit user IDs in colon mode even if the Subject is empty.

* sm/keylist.c (list_cert_colon): Rework listing of user IDs.

Only in colon mode this did not work. Note that an updated libksba is
anyway required to parse a certificate with an empty Subject.

(cherry picked from commit 1067e544c29d652f6f19e47ed2d563e570611e43)

• GnuPG-bug-id: T7171