[Typescript-Fetch] incorrect handling of nullable for additionalProperties

Description

It will create same code for additional properties with nullable and non nullable elements.

Generated code:

export interface EnvVars {
    works?: { [key: string]: string; };
    notworks?: { [key: string]: string; };
}

How I think it should be

export interface EnvVars {
    works?: { [key: string]: string | null; };
    notworks?: { [key: string]: string | null; };
}

Swagger-codegen version

Latest stable 2.2.3 And latest 3 (Tue Nov 14 06:42:44 UTC 2017)

Swagger declaration file content or url

Try to generate with adding somewhere such definitions

definitions:
  envVars:
    description: description
    type: object
    properties:
      works:
        type: object
        additionalProperties:
          type: string
          x-nullable: false
      notworks:
        type: object
        additionalProperties:
          type: string
          x-nullable: true

Command line used for generation

java -jar ./swagger-codegen-cli.jar generate -l typescript-fetch -i ./spec/swagger.yml -DnpmName=client-ts -DnpmVersion=0.1.0 -o ts-gen

Steps to reproduce

Try to generate with provided yaml

Related issues/PRs

Can't find same problem.

Suggest a fix/enhancement

Golang not affected.