[BUG][Kotlin] Data class is not generated anymore

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?
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Description

Our spec used to generated data classes, but now it generates classes without the data keyword. This happens when I have a component that has a allOf with a $ref to a supertype and no extra properties. When I have a component that has a allOf with a $ref to a supertype and an extra property it does generate a data class.

openapi-generator version

Broken in 7.9.0, it did work in 7.8.0

OpenAPI declaration file content or url

components:
  schemas:
    AssetType:
      type: string
      enum:
        - ASSET_A
        - ASSET_B
    BaseAsset:
      type: object
      required:
        - assetId
        - assetType
      properties:
        assetId:
          type: string
          format: uuid
        assetType:
          $ref: '#/components/schemas/AssetType'
      discriminator:
        propertyName: assetType
        mapping:
          ASSET_A: '#/components/schemas/AssetWithoutDataClass'
          ASSET_B: '#/components/schemas/AssetWithDataClass'
    AssetWithoutDataClass:
      allOf:
        - $ref: '#/components/schemas/BaseAsset'
        - type: object          
    AssetWithDataClass:
      allOf:
        - $ref: '#/components/schemas/BaseAsset'
        - type: object
          required:
            - number
          properties:
            number:
              type: string
              example: 'CK1924'

AssetWithDataClass now generates a data class. AssetWithoutDataClass does not generate a data class anymore, but it did in 7.8.0.

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

Start by generating Kotlin models from the YAML specification with OpenAPI Generator 7.8.0 and 7.9.0, focusing on AssetWithoutDataClass and AssetWithDataClass. Trace the Kotlin model-generation entry point to identify the regression for an allOf reference with no extra properties. Done means the no-property subtype again generates a data class while the provided reproduction remains covered by a regression test.