Support jabberd2 SASL with really empty response

Despite http://xmpp.org/rfcs/rfc3920.html#rfc.section.6.2, jabberd version 2.2.14 cannot accept the typical "<response xmlns="urn:ietf:params:xml:ns:xmpp-sasl">=</response>".  Instead it must be truly empty, so we force an empty response for this stanza only.