[Java] Use correct annotation for optional request body

Administrator
2023-03-19T17:21:37Z

*Created by: hinrik*

This adds `required=false` to the `@RequestBody` annotation when the OpenAPI
definition has the body marked as not required.

@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608