Use NULL for nick/blurb in GObject params

As they are only used by gstreamer for gst-inspect & other tools.
Projects like Mutter/gtk have completely dropped them as well, so follow
their path

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3505>

js/ui/windowPreview.js

@@ -29,7 +29,7 @@ const ICON_TITLE_SPACING = 6;
 export const WindowPreview = GObject.registerClass({
     Properties: {
         'overlay-enabled': GObject.ParamSpec.boolean(
-            'overlay-enabled', 'overlay-enabled', 'overlay-enabled',
+            'overlay-enabled', null, null,
             GObject.ParamFlags.READWRITE,
             true),
     },