Allow Passphrase to store empty secrets

Summary: Fixes T6001. We currently don't allow empty secrets, but accounts with no password are occasionally used in the wild.

Test Plan:
  - Created a credential with an empty secret.
  - Revealed secret, saw empty message.
  - Edited it (no form changes), saw secret unchanged.
  - Changed it to a nonempty secret.
  - Revealed nonempty secret.
  - Edited it (no form changes), saw secret unchanged.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6001

Differential Revision: https://secure.phabricator.com/D10414