[TypeScript] Free-Form object being generated incorrectly

Description

I'm trying to generate the typescript models, based on the schemas defined in the YAML file, but there is a model (free-form object) that is not being properly generated and the typescript is generated like

export interface MyModel extends null<String, any> { 
  [key: string]: any;
}

while I was expecting something like:

export interface MylModel {
    [key: string]: any;
}

Is relevant to mention that this problem started happening after the migration from Swagger 2.x version to OpenAPI 3.0 and i also started using this new Swagger-codegen version

Swagger-codegen version

3.0.11

Swagger declaration file content or url

components:
  schemas:
    MyModel:
      type: object
      additionalProperties:
        type: object
      example:
        Sheet 1!A1: 0
        Sheet 1!A2: a text

Command line used for generation

mvn clean generate-resources

Steps to reproduce

Using a valid open API 3.0 YAML file, declare the schema referred above and generate the typescript model using the following pom configuration:

<plugin>
        <groupId>io.swagger.codegen.v3</groupId>
        <artifactId>swagger-codegen-maven-plugin</artifactId>
        <version>${swagger-codegen-maven-plugin.version}</version>
        <executions>
          <execution>
            <phase>generate-sources</phase>
            <goals>
              <goal>generate</goal>
            </goals>
            <configuration>
              <inputSpec>${basedir}/../schema/api.yaml</inputSpec>
              <output>${basedir}/src/app/shared/api</output>
              <language>typescript-angular</language>
              <generateApis>false</generateApis>
              <generateSupportingFiles>true</generateSupportingFiles>
              <generateModelDocumentation>false</generateModelDocumentation>
              <generateModelTests>false</generateModelTests>
              <generateSupportingFiles>false</generateSupportingFiles>
              <generateAliasAsModel>true</generateAliasAsModel>
              <modelPackage>models</modelPackage>
            </configuration>
          </execution>
        </executions>
      </plugin>

Related issues/PRs

Suggest a fix/enhancement

Run the provided mvn clean generate-resources reproduction with the OpenAPI 3 YAML schema and inspect the generated TypeScript model. Compare MyModel with the expected free-form object shape; done means generation no longer emits the invalid null<String, any> extension and produces the index signature shown in the report.