chore(game): overhaul not null annotation usage

[Joltras]

Proposed changes

Currently, the project doesn't use package-info files, which can be used to specify that everything is non-null by default and that things which can be null must be explicitly declared as such. Because of this, there is currently some boilerplate code for handling these annotations.

This pull request updates the annotation handling to reduce this boilerplate and make the codebase more consistent.

Types of changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of
them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before
merging your code.

• I have read the CONTRIBUTING.md
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[github-actions]

Test results

 81 files   81 suites   52s ⏱️
 57 tests  57 ✅ 0 💤 0 ❌
171 runs  170 ✅ 1 💤 0 ❌

Results for commit 8932c43.