Avoid criticals on headless systems

If we don't have a monitor, don't try to use it.
This avoids pointless critical warnings on headless systems.

gtk/gtkwidget.c

@@ -10984,7 +10984,10 @@ gtk_widget_get_scale_factor (GtkWidget *widget)
   display = gtk_widget_get_display (widget);
   monitor = gdk_display_get_monitor (display, 0);
 
-  return gdk_monitor_get_scale_factor (monitor);
+  if (monitor)
+    return gdk_monitor_get_scale_factor (monitor);
+
+  return 1;
 }
 
 /**