Conditional Basic Auth

Previously, basic auth headers were added to every DownloadCache
request.  In the case where there were no basic auth credentials,
empty strings were used for the username and password.  This was
wrong, although tolerated by the primarily used CloudFront.  This
change ensures that basic auth headers are only added when there are
both a username and password specified.

[#72253796]
Merge 72253796-conditional-basic-auth to master

[Completes #72253796]