In either case, when I validated the xml it did not give me any error or warning.
[0] That shouldn't be the case. Both documents should be invalidated with the mention:
"...(organization | person) cannot be empty according to the DTD/Schema"
or something of the kind.
[1] For tns:organization, there is nothing we you do except complying to the schema by adding its children tns:organizationName and tns:organizationId, one each of that order with the correct text types.
[2] For tns:person, you can adding the attribute xsi:nil="true" and the document would be validated. And that is the major aspect of what nullible attribute being true meant in the schema.