Empty string shouldn't be allowed as basic-auth/oauth2 custom password

[kolbinski]

Now, with empty string as a password, such credential is created.

[XVincentX]

Definitely true. Thanks for the report, I’ll look into this.

[XVincentX]

Note to self: this requires a lot of efforts.

1. Make the JSON schema validation on the Admin API
2. Cleanup the credentials schema (requires an huge refactoring effort to make the thing decent)