#24701 Add null type only test to Java okhttp-gson client

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Summary by cubic

Adds a null-only schema property to the Java okhttp-gson 3.1 petstore sample to validate OpenAPI 3.1 type: null handling. This ensures the generator emits a nullable Object field and includes it in equality, hashing, and field validation.

• Spec: adds second_property with type: null and description "just null type".
• Model PetWithTypesObjectNullAndRef: new secondProperty (Object, @javax.annotation.Nullable), included in equals, hashCode, toString, and openapiFields.
• Docs: PetWithTypesObjectNullAndRef now lists secondProperty as optional Object with the description.
• Adds JsonNullable helpers; they are not used and do not change behavior.

Review Notes

• Focus on generated Java for okhttp-gson to confirm type: null maps to a nullable Object and passes validation.
• Regenerate samples to verify outputs stay consistent across generators.