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/search.js

@@ -873,7 +873,7 @@ var SearchResultsView = GObject.registerClass({
             return;
         }
 
-        let from = this._defaultResult ? this._defaultResult : null;
+        const from = this._defaultResult ?? null;
         this.navigate_focus(from, direction, false);
     }