[BUG][Java][Spring] JsonNullable generated for schemas using oneOf/anyOf

[pvdbosch]

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

References to some types are unnecessarily wrapped in JsonNullable, even though nullable: true isn't used. This happens for types generated from schemas that use oneOf or anyOf to list allowed combinations of required properties like this:

      type: object
      properties:
         # ....
      oneOf:
        - required: [propertyA]
        - required: [propertyB]

A workaround for this problem is to add type: object to each of the oneOf subschemas.

openapi-generator version

Since version 7.x including 7.6.0-SNAPSHOT, doesn't occur in 6.x versions.

OpenAPI declaration file content or url

reproducer OpenAPI (+ extract of generated code):
https://gist.github.com/pvdbosch/1f26142fd9df792f7cba6cdff88dbc29

Generation Details

generatorName: spring
inputSpec: src/main/resources/openapi.yaml
additionalProperties:
  useSpringBoot3: true
  useBeanValidation: true
  interfaceOnly: true
  skipDefaultInterface: true
  dateLibrary: java8

Also occurs when using java generator with spring library. Doesn't reproduce for jaxrs-spec generator.

Steps to reproduce

1. Generate code using openapi.yaml
2. generated ReferencingSchema.java has an unneccessary JsonNullable wrapper for its content attribute.

Related issues/PRs

Suggest a fix

My guess is that the generator sets isNullable to true on the schema in this code because the type is missing in the oneOf subschemas ; though I haven't been able to confirm this yet.

This shouldn't be done however because there is a type defined in the schema that includes the oneOf.