Update _helpers.tpl env vars that use secret keys

[bastikus]

Returned default values for the external database

Pull Request

Description of the change

Helm chart does not work when using an external database for NextCloud. "key" is not defined.

Checklist

• [ ] DCO has been signed off on the commit.
• [ ] Chart version bumped in Chart.yaml according to semver.
• [ ] (optional) Variables are documented in the README.md