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