json-schema: treat undefined values as if they were not present.

[brennie]

It would be nice to have the option to treat undefined values as if they were not present, so a schema like

{
  "type": "object",
  "properties": {
    "foo": {
      "type": "string"
    }
  },
  "additionalProperties": false
}

would match objects like {"foo": undefined} in addition to {} (since the foo key will be omitted when the object is serialized to JSON.

As a workaround, I'm doing JSON.parse(JSON.stringify(obj)) when obj may have undefined keys, which is less than ideal.

[cosbgn]

Ideally we would have a coercion option like https://ajv.js.org/coercion.html - It would solve this as well as fix things like "true" -> true, "23" -> 23, undefined -> null, etc