Use shorthand nullability typings for better readability

[jnoordsij]

WHY

BEFORE - What was wrong? What was happening before this PR?

When working on #5732, I encountered some other (nullable) types that could be simplified, to improve readability.

AFTER - What is happening after this PR?

Cleaner code.

HOW

How did you achieve that, in technical terms?

Use shorthand.

Is it a breaking change?

No (or not really); only the change in RegisterUploadEvents::handle is a change in behavior, due to no longer allowing null; however explicit null passing seems unlikely and unwanted here.

How can we test the before & after?

??

If the PR has changes in multiple repos please provide the command to checkout all branches, eg.:

git checkout "dev-branch-name" &&
cd vendor/backpack/crud && git checkout crud-branch-name &&
cd ../pro && git checkout pro-branch-name &&
cd ../../..