[BUG][JAVA] JsonNullable types are not generated for java client models when library is set to 'okhttp-gson' (default) or 'microprofile'

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Related to the next issue: https://github.com/OpenAPITools/openapi-generator/issues/17873

openapi-generator does not generate expected JsonNullable properties when library is set with microprofile nor okhttp-gson(default).

Current output: https://github.com/robertop87/opengen/blob/main/build/generated-src/swagger/src/main/java/org/openapitools/client/model/Pet.java

@SerializedName(SERIALIZED_NAME_TAG)
private String tag; // This must be: JsonNullable<String> tag = JsonNullable.undefined();

Expected output: https://github.com/robertop87/opengen/blob/main/src/test/resources/Pet.java

private JsonNullable<String> tag = JsonNullable.<String>undefined();

openapi-generator version

gradle plugin 7.3.0

OpenAPI declaration file content or url

https://github.com/robertop87/opengen/blob/main/swagger.yml

Generation Details

Using gradle plugin configuration (check the next repository) https://github.com/robertop87/opengen

tasks.register('generateClient', GenerateTask) {
    generatorName.set("java")
    // available libraries list: https://openapi-generator.tech/docs/generators/java/
    // Default: okhttp-gson
    // The next libraries does not generate the expected JsonNullable: okhttp-gson, microprofile
    // The next libraries does generate JsonNullable properly:
    //   jersey2, jersey3, feign, retrofit2, resttemplate, webclient, resteasy, vertx, google-api-client, rest-assured, native, apache-httpclient
    library.set("okhttp-gson") // <-- using default just for testing purposes
    inputSpec.set(swaggerFile.toString())
    outputDir.set(swaggerOutputDir)
    configOptions.set([
            dateLibrary: "java8",
            useJakartaEe: "true",
            openApiNullable: "true" // This is enabled by default
    ])
}

Steps to reproduce

Clone: https://github.com/robertop87/opengen
Run ./gradlew clean assemble

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/17873

Suggest a fix

Enable the generation of JsonNullable types when library configuration is: microprofile or okhttp-gson.

Start with the generateClient Gradle task, swagger.yml, and the generated and expected Pet.java files linked in the report. Run ./gradlew clean assemble using the okhttp-gson and microprofile libraries, then compare their output with the libraries that already generate JsonNullable correctly. Done means both affected libraries produce the expected JsonNullable model properties.