cleanup: Use optional chaining and ?? operator

Those operators have been supported since gjs switched to mozjs78
last cycle. While not ground-breaking, using it makes for a nice
cleanup here and there.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1517>

js/ui/inhibitShortcutsDialog.js

@@ -73,7 +73,7 @@ var InhibitShortcutsDialog = GObject.registerClass({
     }
 
     _buildLayout() {
-        let name = this._app ? this._app.get_name() : this._window.title;
+        const name = this._app?.get_name() ?? this._window.title;
 
         let content = new Dialog.MessageDialogContent({
             title: _('Allow inhibiting shortcuts'),