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/xdndHandler.js

@@ -86,7 +86,7 @@ var XdndHandler = class {
         let dragEvent = {
             x,
             y,
-            dragActor: this._cursorWindowClone ? this._cursorWindowClone : this._dummy,
+            dragActor: this._cursorWindowClone ?? this._dummy,
             source: this,
             targetActor: pickedActor,
         };