gtk_widget_get_scale_factor fallback monitor ignores 0-monitor possiblity

In the below code sample from gtk_widget_get_scale_factor, it is possible to pass in NULL to gdk_monitor_get_scale_factor, causing a segmentation fault. A common scenario where this can occur is with laptop lid-close events.

display = gtk_widget_get_display (widget);
monitor = gdk_display_get_monitor (display, 0);
return gdk_monitor_get_scale_factor (monitor);

Another fallback scenario should be added to just return 1 if no monitors are available.