[BUG] [typescript-axios] Nullable addtional properites are generated as not nullable

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

Additional properties with nullable: true in them are generated as not nullable.

openapi-generator version

6.1.1-SNAPSHOT

OpenAPI declaration file content or url

components:
  schemas:
    MyObject:
      type: object
      required:
        - string
        - data
      properties:
        string:
          $ref: '#/components/schemas/MyString'
        data:
          $ref: '#/components/schemas/MyData'
    MyData:
      type: object
      additionalProperties:
        $ref: '#/components/schemas/MyString'
    MyString:
      type: string
      nullable: true

Generation Details

Generated using the latest typescript-axios generator:

docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:latest generate \
    -i /local/spec/sample.openapi.yaml \
    -g typescript-axios \
    -o /local/out/sample

Generator output

export interface MyObject {
    'string': string | null;
    'data': { [key: string]: string; };
}

As you can see, the regular property is nullable and the additional one is not, even though they're both declared the same way.

Steps to reproduce

Run the typescript-axios generator

Suggest a fix

Make additional properties nullable:

export interface MyObject {
    'string': string | null;
    'data': { [key: string]: string | null; };
}

Start with the typescript-axios generator and reproduce the issue using the provided OpenAPI declaration and Docker command. Compare generated additional-property types with regular nullable properties; done means the generated MyData index signature includes string | null.