Add nullability annotations to entire project

[Proximyst]

I've recently started using idb religiously in my Kotlin projects. Kotlin uses forced nullability and it respects Java (and co.) annotations for nullability and handles it appropriately to force them in the language. It is therefore best for Kotlin users, and standard Java users alike where the IDE can help, to add nullability (@NotNull/@Nullable) wherever possible.

[NoJokeFNA]

Feel free to create a PR for that