[BUG] Example responses return undefined

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?

• Have you validated the input using an OpenAPI validator (example)?

• What's the version of OpenAPI Generator used?

• Have you search for related issues/PRs?

• What's the actual output vs expected output?

• [Optional] Bounty to sponsor the fix (example)

Description

There are two related issues:

1. When generating code that is nullable: true, the example response is undefined
2. If an optional parameter is left out of an example response, it has an undefined value.

openapi-generator version

"@openapitools/openapi-generator-cli": "^1.0.13-4.3.1",

OpenAPI declaration file content or url

components:
  schemas:
    Animals:
      description: 'Returns cat and/or dog'
      type: object
      properties:
        id:
          type: string
          format: uuid
        dog:
          type: string
          nullable: true
        cat:
          type: string
          nullable: true
      required: ['id']


responses:
  RetrievedAnimals:
    description: 'Retrieved Animals'
    content:
      application/json:
        schema:
          $ref: '#/components/schemas/Animals'
        example:
          id: '64acc332-382e-46f3-b060-737928686f60'
          dog: null
 

Current output:

{
  id: '64acc332-382e-46f3-b060-737928686f60',
  dog: undefined,
  cat: undefined
}

Expected output:

{
  id: '64acc332-382e-46f3-b060-737928686f60',
  dog: null,

}

Suggest a fix

#isNullable does not seem to be utilized.
I suggest returning response.json(); in all instances.

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache#L59-L90

Start with modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache and reproduce the issue with the Animals schema in the report. Check how nullable and omitted optional properties are rendered in the example response. Done means nullable values remain null and omitted optional properties are not emitted as undefined.