Resolver method signatures

[Cantor, Scott]

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?

-- Scott