loginDialog: Fix timing issue

Fix the first task of the timed login batch not returning the hold on
`_waitForItemForUser`.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2116>

js/gdm/loginDialog.js

@@ -1059,9 +1059,10 @@ var LoginDialog = GObject.registerClass({
         let tasks = [
             () => {
                 if (this._disableUserList)
-                    return;
+                    return null;
 
                 this._timedLoginUserListHold = this._waitForItemForUser(userName);
+                return this._timedLoginUserListHold;
             },
 
             () => {