key file: Handle filename being NULL

This happens when we are default-constructed
without explicit arguments.

Closes: https://gitlab.gnome.org/GNOME/glib/issues/1825

gio/gkeyfilesettingsbackend.c

@@ -740,6 +740,7 @@ g_keyfile_settings_backend_set_property (GObject      *object,
     case PROP_FILENAME:
       /* Construct only. */
       g_assert (kfsb->file == NULL);
+      if (g_value_get_string (value))
         kfsb->file = g_file_new_for_path (g_value_get_string (value));
       break;