Don't prompt to upgrade unset passwords

Summary:
Fixes T4463. When your VCS or account password is not set, we test it for upgrade anyway. This doesn't make sense and throws shortly into the process because the empty hash isn't parseable.

Instead, only show upgrade prompts when the password exists.

Test Plan:
  - Added a password to an existing account with no password via password reset.
  - Added a VCS password to an existing account with no VCS password.
  - Observed no fatals / nonsense behaviors.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T4463

Differential Revision: https://secure.phabricator.com/D8282