[typescript-axios] additionalProperties generates null interface

Description

The usage of additionalProperties generates invalid code that tries to extend a null interface. The output looks like this:

export interface ExampleRequestBody extends null<String, any> {
}

The null reference needs to be manually changed to a Map<String, any> in order to compile

All of the following usages are not working:

additionalProperties: true

additionalProperties:
  type: string

additionalProperties:
  anyOf:
    - type: string
    - type: boolean

Swagger-codegen version

swagger-codegen-cli-v3:3.0.35

Swagger declaration file content or url

openapi: "3.0.3"
info:
  title: "Custom Test Api"
  version: "0.0.0"
components:
  schemas:
    ExampleRequestBody:
      type: "object"
      additionalProperties: true
paths:
  /example/request:
    post:
      requestBody:
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/ExampleRequestBody"
      responses:
        204:
          description: "Success"

Command line used for generation

docker run --rm -v ${PWD}:/local --user=`id -u`:`id -g` swaggerapi/swagger-codegen-cli-v3:3.0.35 generate -i /local/open-api-spec.yaml -l typescript-axios -o /local/out

Steps to reproduce

1. Pull the correct docker image:

docker pull swaggerapi/swagger-codegen-cli-v3:3.0.35

2. Copy the .yaml file above
3. Run the command above with the correct path to the .yaml file

Start by reproducing the generated TypeScript client with the supplied OpenAPI YAML and Docker command, then trace the typescript-axios generator output for additionalProperties. Done means the generated interface is valid TypeScript and all three additionalProperties forms produce compilable output without a null interface.