Properly apply nullable validation

[dvdheiden]

Closes #37

Changes

Fixed

• Properly apply nullable validation in BorgRepository, DatabaseUser and PassengerApp models.

Checks

• [x] The version constant is updated in Client.php
• [x] The changelog is updated (when applicable)
• [x] The supported Cluster API version is updated in the README (when applicable)

[mbardelmeijer]

Thanks!