Don't throw when user tries to use an empty password via HTTP auth
Summary: Fixes T4064. See discussion there.
Test Plan: Tried git clone http://... with empty password, got 403. Retried with actual password, got a clone.
Reviewers: jamesr, btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T4064
Differential Revision: https://secure.phabricator.com/D7508