wwan: Ensure we get existing objects on reset

When ModemManager become available, NetworkManager resets
GDBusObjectManagerClient object.

But there is a race condition if object-added is emitted before
modm_ensure_manager(), we need to check existing objects if we want to be
in sync with ModemManager.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1957

src/core/devices/wwan/nm-modem-manager.c

@@ -265,9 +265,7 @@ modm_handle_name_owner_changed(MMManager *modem_manager, GParamSpec *pspec, NMMo
     modm_clear_manager(self);
     modm_ensure_manager(self);
 
-    /* Whenever GDBusObjectManagerClient is fixed, we can just do the following:
-     * modm_manager_available (self);
-     */
+    modm_manager_available (self);
 }
 
 static void