Fix a warning about labels not being GObjects when there isn't a label (when the menu item is a separator).
Fix a warning about labels not being GObjects when there isn't a label (when the menu item is a separator).

sharoonthomas/gtk-mac-integration,GNOME/gtk-mac-integration,sharoonthomas/gtk-mac-integration,sharoonthomas/gtk-mac-integration,jralls/gtk-mac-integration,GNOME/gtk-mac-integration,sharoonthomas/gtk-mac-integration,sharoonthomas/gtk-mac-integration,jralls/gtk-mac-integration,jralls/gtk-mac-integration,GNOME/gtk-mac-integration