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/status/system.js

@@ -26,7 +26,7 @@ const SHOW_BATTERY_PERCENTAGE = 'show-battery-percentage';
 
 const PowerToggle = GObject.registerClass({
     Properties: {
-        'fallback-icon-name': GObject.ParamSpec.string('fallback-icon-name', '', '',
+        'fallback-icon-name': GObject.ParamSpec.string('fallback-icon-name', null, null,
             GObject.ParamFlags.READWRITE,
             ''),
     },