Resolver method signatures

[Tom Zeller]

> When I first started applying Null/Nonnull annotations to the older code,
> I made the Resolver methods take a Nullable CriteriaSet because there was
> code written that assumed that.
>
> I don't know if that was deliberate, but I don't particularly like that,
> since it creates extra null checking work to avoid runtime errors. Is it
> ok to just change that assumption and require an empty CriteriaSet instead
> of null?

+1