_expected_server_name is null when use_tls is set and anonymous connection enabled.

[ergin-ozekes]

If the anonymous connection enabled to xmpp server(openfire 3.9.3) and ssl is enabled the ceritificate verification can not established due to _expected_server_name is null. In the sleekxmpp/xmlstream/xmlstream.py file at the bottom of the connect function if we add below lines before return statement this issue can be fixed.

if connected and self._expected_server_name == '':
self._expected_server_name = host

I've discussed this case with Thomas Calmant and he sad that "you should share this behaviour with the sleekxmpp community"