overviewControls: Don't update work area box when headless

When headless, we don't have a monitor we can derive a size from, so
don't even try, just let it be updated when we actually have a monitor
that will show stuff.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5831
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2478>

js/ui/overviewControls.js

@@ -55,6 +55,9 @@ class ControlsManagerLayout extends Clutter.BoxLayout {
 
     _updateWorkAreaBox() {
         const monitor = Main.layoutManager.primaryMonitor;
+        if (!monitor)
+            return;
+
         const workArea = Main.layoutManager.getWorkAreaForMonitor(monitor.index);
         const startX = workArea.x - monitor.x;
         const startY = workArea.y - monitor.y;