[typescript-fetch] - Fix Nullable Types

[l0gicgate]

#17798 introduced a breaking change where the distinction between undefined and nullable was lost.

The original contributor of this PR explained his rationale here for doing this, which is wrong:
#17798 (comment)

Take the instance where we do a PATCH request and I want to partially update an entity. That means I would be able to pass in a partial object with subset of the fields.

Here's a definition of what a patch could look like, you can optionally pass any of the parameters. In this case, the subTitle field is also nullable, which would make it null in the database if I wanted to erase it:

export interface ArticlePatch {
    title?: string;
    subTitle?: string | null;
}

Scenario 1: Patching title only, I don't pass in subTitle

PATCH /articles/1
{
  "title": "My title",
}

Scenario 2: Patching subTitle only, making it null. Now I can't do this, because the types are broken:

PATCH /articles/1
{
  "subTitle": null,
}

subTitle is nullable: true in swagger.json:

{
  "subTitle": {
    "type": "string",
    "nullable": true
  }
}

In previous versions 7.3.0 and down the output was this:

export interface ArticlePatch {
    title?: string;
    subTitle?: string | null;
}

Since this PR has been merged:

export interface ArticlePatch {
    title?: string;
    subTitle?: string; // No longer nullable
}

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• File the PR against the correct branch: master (upcoming 7.6.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.
  @TiFu (2017/07) @taxpon (2017/07) @sebastianhaas (2017/07) @kenisteward (2017/07) @Vrolijkx (2017/09) @macjohnny (2018/01) @topce (2018/10) @akehir (2019/07) @petejohansonxo (2019/11) @amakhrov (2020/02) @davidgamero (2022/03) @mkusaka (2022/04)

[macjohnny]

Thanks for the fix and the comprehensible explanation!