Some null annotations and javadoc fix

While searching how to work with queues...
These should all be `@CheckForNull`.

In general `@Nullable` should only be used in special cases where you are giving up on static checks because there are more complex considerations that would let a human prove that part of a program is correct even though a static analysis tool would think it possible for there to be a `NullPointerException`; and where adding in (technically superfluous) null checks in all cases would be onerous.