Do not require authzid on SASL EXTERNAL for S2S

The XMPP specifications in RFC 6120, section 6.3.8. “Authorization Identity” specify that during SASL negotiation, an authorization identity is not required:

An authorization identity is an OPTIONAL identity included by the initiating entity to specify an identity to act as (see Section 2 of [SASL]).

Openfire currently requires an authorization identity (that matches the stream’s “from” attribute value) to be provided. It should not.