Handle clients that don't accept auth_url=None

Some clients fail when passed None as the auth_url. Pass them an empty
string instead when the auth_url is None.

Change-Id: Ie7cf788a00660efae578b6444ee231876025ebba
Closes-Bug: #1332991
Closes-Bug: #1334492
Merge "Handle clients that don't accept auth_url=None"