radio-menu-item: Allow arguments to be NULL

radio-menu-item: Allow arguments to be NULL

Some arguments, like the group and the label of a RadioMenuItem, can be
NULL: the RadioMenuItem has all the code to deal with them. The argument
validation is too strict, though, for instance doing:

  return_if_fail (IS_RADIO_MENU_ITEM (foo))

  if (foo != NULL)
    set_foo (foo)

Which is obviously incorrect.

This commit also modifies the annotations of the API, to ensure that
language bindings do the right thing.

https://bugzilla.gnome.org/show_bug.cgi?id=671362