Check attributes on null objects, and fill in unknowns. If we're
evaluating the object, it either means we are at the top level, or a
NestingSingle block was present, and in either case we need to treat the
attributes as null rather than the entire object.
Switch on the block types rather than Nesting, so we don't need add any
logic to change between List/Tuple or Map/Object when DynamicPseudoType
is involved.