[BUG][JAVA] Client generator ignores 'nullable' property

Description

Client generator ignores 'nullable' property of the object schema. Instead, javax.annotation.Nullable generation seems to be based on 'required' property. This behaviour is incorrect - required fields have to be provided, but not necessarily non-null, i.e { "field" : null } satisfies the 'required'.

Nullable is part of OpenAPI 3.0 spec. To make it more robust, x-nullable could be handled, too.

OpenAPI declaration file content or url

"components": {
   "schemas": {
      "SomeEntity": {
        "type": "object",
        "properties": {
          "name": {
            "id": "string",
            "nullable": false
          }
      }
   }
}

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/3264

Start by tracing the Java client generator's schema handling for the OpenAPI declaration shown, especially how nullable and required are converted into javax.annotation.Nullable. Compare the behavior with the linked issue and OpenAPI 3.0 nullable semantics. Done means generated Java clients distinguish nullability from whether a property is required, with x-nullable considered if supported.