Management notifications show _AMQ_User = null when connection is authenticated via certificate in AMQ 7

Issue

• _AMQ_User = null, when connection is authenticated via TextFileCertificateLoginModule.
• Issue can be replicated using the example configuration ssl-enabled-dual-authentication and management-notifications.

Received notification:
  _AMQ_Address: exampleQueue
  _AMQ_NotifType: CONSUMER_CREATED
  _AMQ_RoutingName: exampleQueue
  _AMQ_NotifTimestamp: 1537152205334
  JMSXDeliveryCount: 1
  _AMQ_SessionName: 731b711a-ba23-11e8-928d-68f72828e01c
  _AMQ_ClusterName: exampleQueuef6c05ece-b94e-11e8-a626-e8b1fcafa0d2
  _AMQ_User: null
  _AMQ_ConsumerCount: 1
  _AMQ_RemoteAddress: /127.0.0.1:59984
  _AMQ_Distance: 0