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.

src/shared/varlink.c

@@ -221,11 +221,11 @@ DEFINE_PRIVATE_STRING_TABLE_LOOKUP_TO_STRING(varlink_state, VarlinkState);
         log_debug("%s: " fmt, varlink_server_description(s), ##__VA_ARGS__)
 
 static inline const char *varlink_description(Varlink *v) {
-        return strna(v ? v->description : NULL);
+        return (v ? v->description : NULL) ?: "varlink";
 }
 
 static inline const char *varlink_server_description(VarlinkServer *s) {
-        return strna(s ? s->description : NULL);
+        return (s ? s->description : NULL) ?: "varlink";
 }
 
 static void varlink_set_state(Varlink *v, VarlinkState state) {