status/brightness: Hide when proxy has no owner

Properties are null while a proxy has no owner. Javascript helpfully
coerces that into `Number(null)` (a.k.a. 0), so we end up with a
broken slider.

Explicitly check that the value is an integer before doing the
comparison to catch that case.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2391>

js/ui/status/brightness.js

@@ -67,7 +67,8 @@ class Indicator extends PanelMenu.SystemIndicator {
     }
 
     _sync() {
-        let visible = this._proxy.Brightness >= 0;
+        const brightness = this._proxy.Brightness;
+        const visible = Number.isInteger(brightness) && brightness >= 0;
         this._item.visible = visible;
         if (visible)
             this._changeSlider(this._proxy.Brightness / 100.0);