varlink: say "varlink:" instead of "n/a:" when no description is available

For new connections, we log something like this:

systemd[1]: n/a: New incoming connection.
systemd[1]: n/a: Connections of user 997: 0 (of 1024 max)
systemd[1]: varlink-22: varlink: setting state idle-server
systemd[1]: varlink-22: New incoming message: ...

This "n/a" is not very pretty, and without context it would be hard to even
figure out this is a varlink connection.