chore: migrate to TypeScript strict in Payload package - Part 2/N

[GermanJablo]

Important: An intentional effort is being made during migration to not modify runtime behavior except in very rare exceptions. This implies that there will be several assertions, non-null assertions, and @ts-expect-error. This philosophy applies only to migrating old code to TypeScript strict, not to writing new code. For a more detailed justification for this reasoning, read this comment.