Awakeable handle annotated with @NotNull but needs null for VOID types

[StephanEwen]

For VOID typed awakeables, the value passed during resolution is null but the handler is annotated to not accept that.

[slinkydeveloper]

Unfortunately I don't really have a good solution that solves it with the nullness annotations. I see two ways of solving this:

* Either have a variant resolve() without arguments.
* Or have some marker unit value (a-la kotlin Unit), e.g. Unit.INSTANCE or something like that.

I think I'll go with the former

[slinkydeveloper]

Perhaps this @NonNull is indeed too restrictive to begin with. I'll remove it for the time being.

[slinkydeveloper]

I've figured that with JSpecify I can enforce this directly on the Serde annotation, and not on the individual context methods. Working on this now.