GtkRadioButtons: Mark group arguments as "allow-none"

GtkRadioButtons: Mark group arguments as "allow-none"

In the current introspection data, the various gtk_radio_button_new_*
constructors take an existing group or widget. These all allow passing a NULL
widget or group list, so that it becomes easy to build a radio button list in a
loop (pass in None the first time, and the previously created button in all
other iterations).

Add the missing "allow-none" tag to the introspection data, so that this will
also eventually work for GIR users. NB that it does not actually work for some
constructors yet (like new_with_label_from_widget()) due to
https://bugzilla.gnome.org/show_bug.cgi?id=561264.

https://bugzilla.gnome.org/show_bug.cgi?id=635253