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/backlight.js

@@ -21,7 +21,7 @@ const BrightnessProxy = Gio.DBusProxy.makeProxyWrapper(BrightnessInterface);
 const SliderItem = GObject.registerClass({
     Properties: {
         'value': GObject.ParamSpec.int(
-            'value', '', '',
+            'value', null, null,
             GObject.ParamFlags.READWRITE,
             0, 100, 0),
     },
@@ -60,11 +60,11 @@ const SliderItem = GObject.registerClass({
 const DiscreteItem = GObject.registerClass({
     Properties: {
         'value': GObject.ParamSpec.int(
-            'value', '', '',
+            'value', null, null,
             GObject.ParamFlags.READWRITE,
             0, 100, 0),
         'n-levels': GObject.ParamSpec.int(
-            'n-levels', '', '',
+            'n-levels', null, null,
             GObject.ParamFlags.READWRITE,
             1, 3, 1),
     },