fix: allow extended identifier with empty administrative-domain

Extended identifier are allowed to have an empty administrative-domain
attribute in the encoded XML, see [1].

Extended identifier are not allowed to have an administrative-domain
in the enclosing identity (not even an empty one). At the moment
this is only guaranteed by the factory methods of
Identifiers.createExtendedIdentity().

[1]: TNC IF-MAP Binding for SOAP, Specification Version 2.1, Revision 15, 7 May 2012.