fix: update typing for not-yet published stories

[finnboeger]

Pull request type

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Other (please describe):

How to test this PR

Create a new story in the root directory (and only save it, don't publish it) and access its draft version via the API. The response should contain the following null values that are not yet reflected in the typing:

{
   "story":{
      ...
      "parent_id": null,
      ...
      "first_published_at":null,
      "release_id":null,
      ...
      "default_full_slug":null,
      "translated_slugs":null
   },
   ...
}

What is the new behavior?

fields can now be null according to the typing

Other information

I've missed those fields in #763 as I only checked for published stories, this should now cover all story states.

It may be a good idea to split between Published/Non-Published state but that's probably something for usages of the library as it would be a big change.