ATS: Use default CLR when field does not exist on server

When field does not exist on server but does in POCO, use the default CLR type.
Looks like it could derive from existing ObjectArrayValueReader. Also, it shouldn't be necessary to check IsNull in ReadValue since the caller is supposed to call IsNull and not call ReadValue if IsNull returns true. You could add an Assert instead. Also it is possible that the original intentions of how IValueReader is used may not hold anymore.