fix: explicitly define more nullable types

[zrckr]

Some fields were already marked as nullable in b019045 and 97c5531. This PR marks additional fields as nullable and adds default values for the fields, in accordance with the class definitions in the original game. This also adds the corresponding tests.

I’m not entirely sure this will completely eliminate ALL footguns for FEZEditor or HAT mod loader, but at least it makes the contract more explicit, given that the game predates nullable-reference annotations.

If there’s a better way to resolve the root discrepancy, feel free to reject this PR.

[Krzyhau]

If you're at least 90% sure your manual checks for creating those unit tests are correct, I'm perfectly fine with merging it. However, you've shown interest in producing unit tests directly from stripped game libraries. So I'll let you choose what's more appealing to you - wouldn't want to take away fun from you :)

[zrckr]

I used the updated FezEngine.dll containing readers' logic from the FezEngine.Readers namespace. I created a new test that reads this IL, which we use to verify the Repacker's types. This was done because, ultimately, these are the readers that will be used to read the assets, whether they're in XNB or JSON format.