Socket's implementation of ref_state_set to return empty sets instead of NULL

Socket's implementation of ref_state_set to return empty sets instead of NULL

Calls to atk_object_ref_state_set() are expected to return a valid
state set always, so it would be better if this implementation of
socket's ref_state_set() returned empty sets instead of just NULL, to
avoid calling to atk_state_set_*() functions over invalid instances of
AtkStateSet (e.g. add_pending_items() in accessible-cache.c).

https://bugs.freedesktop.org/show_bug.cgi?id=42350