gio-tool: Do not print settable arguments unless they are any
gio-tool: Do not print settable arguments unless they are any

"Settable arguments:" is printed even if they are not any arguments
to print. Do not print it similarly as it is done for "Writable
namespaces:".

https://bugzilla.gnome.org/show_bug.cgi?id=776169

johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib