gdbus/client: Don't GetManagedObjects w/o handlers

The client code currently issues GetManagedObjects if new handlers are
set via g_dbus_client_set_proxy_handlers. An application may set these
to NULL before unref'ing a client or to simply prevent further events.
Hence, there is no need to refresh objects or properties if all handlers
are NULL.
diff --git a/gdbus/client.c b/gdbus/client.c
index 238b348..cd5c767 100644
--- a/gdbus/client.c
+++ b/gdbus/client.c
@@ -1374,7 +1374,8 @@
 	client->property_changed = property_changed;
 	client->user_data = user_data;
-	get_managed_objects(client);
+	if (proxy_added || proxy_removed || property_changed)
+		get_managed_objects(client);
 	return TRUE;