Don't let selector POST be fooled by 'empty' values

When the 'empty' type is an object, it specifically wants to auto-vivify
and not just get empty placeholder objects.
Only literal values can be empty

The way this check worked was badly broken when 'empty' was set to a class,
and this was the same class as 'isa'.
JsonProperty: default 'empty' to 'isa'

The intent here is to be able to quickly test parsed JSON documents without
having to check that the intermediate objects were defined.  Instead of
throwing AttributeError, it returns empty objects, which will test false.