ATNA: provide a better value in ActiveParticipant[source].userId

Currently, "unknown" is set.

```xml
<ActiveParticipant UserID="unknown" AlternativeUserID="1" UserIsRequestor="false" NetworkAccessPointID="172.16.0.90" NetworkAccessPointTypeCode="2">
    <RoleIDCode csd-code="110153" codeSystemName="DCM" originalText="Source Role ID" />
  </ActiveParticipant>
```