extensionBase: Fix issue in lookupByURL()

The lookupByUUID() implementations in the subtypes return null
when the lookup failed, not undefined.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2879>

js/extensions/sharedInternals.js

@@ -28,7 +28,7 @@ export class ExtensionBase {
 
             const dirName = GLib.path_get_basename(path);
             const extension = this.lookupByUUID(dirName);
-            if (extension !== undefined)
+            if (extension !== null)
                 return extension;
         } while (path !== '/');