Don't add empty `required` properties information in DTO fields

[duharound]

In the OpenAPI spec at /api-docs/oas-3.0.0.json, the DTO schema definitions all include an empty required field like

"required": []

This creates errors with some automated schema checks as for example used in https://github.com/openapi-generators/openapi-python-client, such that one has to either remove the required field or add manually some required properties.

Related to #42