logind: fix emission of PropertiesChanged on seats

The story is the same as in 471cffcfb0:
device_attach() → seat_send_changed() → sd_bus_emit_properties_changed_strv()
→ emit_properties_changed_on_interface() → node_vtable_get_userdata()
→ seat_object_find(), which returns 0 because message == NULL.
But when we are emitting a signal, message is always NULL. Removing the
overeager check and assert in the called function allow the signal to be
emitted.

Fixes #13769.

src/login/logind-dbus.c

@@ -189,7 +189,6 @@ int manager_get_seat_from_creds(
         int r;
 
         assert(m);
-        assert(message);
         assert(ret);
 
         if (SEAT_IS_SELF(name) || SEAT_IS_AUTO(name)) {

src/login/logind-seat-dbus.c

@@ -280,8 +280,6 @@ int seat_object_find(sd_bus *bus, const char *path, const char *interface, void
                 return -ENOMEM;
 
         message = sd_bus_get_current_message(bus);
-        if (!message)
-                return 0;
 
         r = manager_get_seat_from_creds(m, message, e, error, &seat);
         if (r == -ENXIO) {