Improve JSON schema validation for non-required fields

[JohannesOehm]

Currently, the JSON schema validation in the backend is not consistent with the cardinality required in the openEHR-Template. Elements, which may be missing in the openEHR-Template, lead to JSON schema validation errors.

Unfortunatly, by default, those errors are never shown in neither the frontend or any of the logs. Please improve also the logging behaviour.