QPID-5240: Change ExternalSaslServer to avoid NPE possibility.

Contract for SaslServer#getAuthorizationID does not disallow the calling of getAuthorizationID following a failed authentication
so returning null in this case seems reasonable (com.sun.security.sasl.CramMD5Server behaves in this way).

Also refactored EAMT to have small tightly targetted tests.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1536126 13f79535-47bb-0310-9956-ffa450edef68