Infinite auth loop if empty credentials are specified in content source URL and repository requires auth

If a repository requires auth (i.e., is private) and there are empty credentials specified in the content source URL. For example:

content:
  sources:
  - url: https://@github.com/octocat/private-repo

This loop happens because Antora keeps sending the empty credentials instead of looking up new credentials in the credential manager store. If the username is not specified on the credentials, Antora should assume there's nothing to send and delegate to the credential manager store.