Handle empty auth username to ensure valid CLF formatting in access logging

Hi Karl,

When the auth username is not specified by the source client, this results in an empty string which causes a log line where the second dash (-) is missing. The access.log line is no longer CLF compliant and cannot be parsed correctly.

This patch ensures that dash (-) is used for auth username in this case.

Can you please merge this into icecast-kh too?

Kind regards,
Klaas Jan