sip_im_session2.py: make sure history_file is always initialized

darcs-hash:20090317100631-b2873-651370987140c635d4a73bd7d1b3c69a289e868f
sip_im_session2.py: allow relay to be None

darcs-hash:20090317100659-b2873-6779cd9dceb54d928422a9ba91bebade65caae00