Engine-XMPP: regression from fc69d82cead caused NRE when sending messages to bare Jids

this was noticed in gtalk, as messages there are always sent to bare Jids.
calling /whois on a contact to which such a message had been sent caused the NullRefException.
this will be properly fixed in refactor/XmppResourceModel
Merge pull request #155 from oli-obk/fix/regression_send_message