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/panel.js

@@ -143,8 +143,8 @@ class AppMenu extends PopupMenu.PopupMenu {
 
         this._updateWindowsSection();
 
-        let appInfo = app ? app.app_info : null;
-        let actions = appInfo ? appInfo.list_actions() : [];
+        const appInfo = app?.app_info;
+        const actions = appInfo?.list_actions() ?? [];
 
         this._actionSection.removeAll();
         actions.forEach(action => {