logind: fix emission of PropertiesChanged for users

The same as parent commit, but users. This is the third and last
foo_object_find() function in logind, so I think that this particular
family of bugs is finally squashed.

src/login/logind-dbus.c

@@ -163,7 +163,6 @@ int manager_get_user_from_creds(Manager *m, sd_bus_message *message, uid_t uid,
         User *user;
 
         assert(m);
-        assert(message);
         assert(ret);
 
         if (!uid_is_valid(uid))

src/login/logind-user-dbus.c

@@ -244,8 +244,6 @@ int user_object_find(sd_bus *bus, const char *path, const char *interface, void
                 sd_bus_message *message;
 
                 message = sd_bus_get_current_message(bus);
-                if (!message)
-                        return 0;
 
                 r = manager_get_user_from_creds(m, message, UID_INVALID, error, &user);
                 if (r == -ENXIO) {