[BUG][typescript-fetch] Optional array properties containing non-primitive elements produce TypeError on JSON serialization

Created by: Dijky

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

When an object property ...

• is not required and
• is an array of non-primitive elements (e.g. array of objects) and
• is serialized to JSON (e.g. when used in a request body) and
• is not set/set to undefined (i.e. omitted)

... then the generated {{classname}}ToJSON function will produce a TypeError on run-time because Array.prototype.map() is called on the undefined property.

Runtime Error Message

TypeError: value.replacements is undefined

openapi-generator version

Tested on v4.0.0-beta2. No relevant changes have been made since then.

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Test API
  version: 0.0.1
paths:
  /someting:
    post:
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/RequestBody'
      responses:
        '200':
          description: OK
components:
  schemas:
    RequestBody:
      type: object
      properties:
        replacements:
          type: array
          items:
            type: object
            properties:
              key:
                type: string
              value:
                type: string

Command line used for generation

openapi-generator-cli -g typescript-fetch -i openapi.yml -o src/api/

Steps to reproduce

1. Define a property on an object as demonstrated.
2. Use the object as the schema for a JSON request body.
3. Generate client code with typescript-fetch.
4. Call the API method with the property in question left undefined.

Related issues/PRs

This bug was introduced with the refactoring in #569.
I'm aware that #802 (closed) will probably deprecate this eventually.

Suggest a fix

A PR will follow soon.

Idea: The call to Array.prototype.map() needs to be guarded against undefined.
This is already done on other optional properties such as Dates, as well as on JSON deserialization.