All the apache log examples I can find use a - dash if the remote_user is unknown.
http://www.monitorware.com/en/logsamples/apache.php
http://ossec-docs.readthedocs.io/en/latest/log_samples/apache/apache.html
https://kb.webtrends.com/articles/Information/Sample-Log-File-Apache-Extended-Log-File-Format
https://raw.githubusercontent.com/aagea/elk-example/master/apache_logs