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/appDisplay.js

@@ -972,7 +972,7 @@ class AppDisplay extends BaseAppView {
         this._switcherooProxy = global.get_switcheroo_control();
         if (this._switcherooProxy) {
             let prop = this._switcherooProxy.get_cached_property('HasDualGpu');
-            discreteGpuAvailable = prop ? prop.unpack() : false;
+            discreteGpuAvailable = prop?.unpack() ?? false;
         } else {
             discreteGpuAvailable = false;
         }