R1:a6296a64a457

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…