[java][model] Collection initialized to empty collection even if not required

Created by: jmini

We are trying to use the same model between our client (using a model serialized with Jackson) and server (JaxRS jersey2).

If a collection is not required, the server model set its value to null.

https://github.com/OpenAPITools/openapi-generator/blob/7f36f2642209c6d7070e1d76209ba1928d816922/modules/openapi-generator/src/main/resources/JavaJaxRS/pojo.mustache#L24-L29

This is not the case on client side.

https://github.com/OpenAPITools/openapi-generator/blob/7f36f2642209c6d7070e1d76209ba1928d816922/modules/openapi-generator/src/main/resources/Java/pojo.mustache#L61

In my opinion this should be aligned.