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/panel.js

@@ -260,11 +260,11 @@ const AppMenuButton = GObject.registerClass({
 
 const WorkspaceDot = GObject.registerClass({
     Properties: {
-        'expansion': GObject.ParamSpec.double('expansion', '', '',
+        'expansion': GObject.ParamSpec.double('expansion', null, null,
             GObject.ParamFlags.READWRITE,
             0.0, 1.0, 0.0),
         'width-multiplier': GObject.ParamSpec.double(
-            'width-multiplier', '', '',
+            'width-multiplier', null, null,
             GObject.ParamFlags.READWRITE,
             1.0, 10.0, 1.0),
     },