Don't parse non-existing arguments

The definition of the signal declares 0 arguments, so the listener can't
assume there are any.

Found by Michael Stahl via this valgrind snippet:

==4770== Conditional jump or move depends on uninitialised value(s)
==4770==    at 0x3104C10812: text_selection_changed_event_listener
(event.c:1036)
==4770==    by 0x30EF42160A: signal_emit_unlocked_R (gsignal.c:3519)
==4770==    by 0x30EF42A180: g_signal_emit_valist (gsignal.c:3309)
==4770==    by 0x30EF42A8F9: g_signal_emit_by_name (gsignal.c:3405)
==4770==    by 0x1835EE96:
AtkListener::notifyEvent(com::sun::star::accessibility::AccessibleEventObject
const&) (atklistener.cxx:454)
==4770==    by 0x56E46BA:
comphelper::AccessibleEventNotifier::addEvent(unsigned int,

https://bugzilla.gnome.org/show_bug.cgi?id=741734