encode:      [{"kind":"answer","text":"a dog","yes":nothing,"no":nothing}]
json_encode: [{"kind":"answer","text":"a dog","yes":null,"no":null}]

json_encodable, with nothing in it: true
json_encodable, with unknown in it: false

and json_encode itself:
runtime error at 07_not_json.bas:18:3: json_encode: `unknown` has no JSON representation; omit the field or replace it with nothing (null)
