Improve nullability for methods in the api module

[lukstbit]

Purpose / Description

Ran through the public api methods of the API module and checked the nullability of the methods and made some changes. Each commit features a method change with some comments in the commit message.

The last commit removes the todo about nullability, but I can remove it if anyone else wants a go at checking the nullability.

Fixes

Fixes? #12310

How Has This Been Tested?

Ran all checks.

Checklist

• [x] You have a descriptive commit message with a short title (first line, max 50 chars).
• [x] You have commented your code, particularly in hard-to-understand areas
• [x] You have performed a self-review of your own code
• [ ] UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• [ ] UI Changes: You have tested your change using the Google Accessibility Scanner