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>

src/shell-window-tracker.c

@@ -105,9 +105,7 @@ shell_window_tracker_class_init (ShellWindowTrackerClass *klass)
   gobject_class->finalize = shell_window_tracker_finalize;
 
   props[PROP_FOCUS_APP] =
-    g_param_spec_object ("focus-app",
-                         "Focus App",
-                         "Focused application",
+    g_param_spec_object ("focus-app", NULL, NULL,
                          SHELL_TYPE_APP,
                          G_PARAM_READABLE | G_PARAM_STATIC_STRINGS);