[typescript] Don't throw exception when `additionalProperties` is set to `true`

Created by: ty-v1

resolves #12973 (closed)

This PR enables typescript generator to generate code without throwing any exceptions when additionalProperty is explicitly set to true, as in the following example.

openapi: 3.0.3
components:
  schemas:
    foo:
      properties:
        bar:
          additionalProperties: true
          description: baz
          type: object
      type: object
info:
  title: Repro
  version: 1.0.0
paths:
  "/":
    get:
      description: Foo
      operationId: getFoo
      responses:
        '200':
          content:
            application/json:
              schema:
                properties:
                  foo:
                    description: Foo.
                    type: string
                required:
                - foo
                type: object
          description: Foo
      summary: Foo
      tags:
      - metadata

PR checklist

• Read the contribution guidelines.
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  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh

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