Fields on `Schema` should be nullable

[bombsimon]

I don't find this statement in the RFC but given this error in the jtd Rust crate and the fact that a Schema is an enum and not a struct on the Rust side you can't have both Type and Enum on a schema.

Given I marshal the schema jtd.Schema{Enum: []string{"foo", "bar"} I will get the following output:

{
  "definitions": null,
  "metadata": null,
  "nullable": false,
  "ref": null,
  "type": "",
  "enum": [
    "foo",
    "bar"
  ],
  "elements": null,
  "properties": null,
  "optionalProperties": null,
  "additionalProperties": false,
  "values": null,
  "discriminator": "",
  "mapping": null
}

If I send this schema to a Rust application to be deserialized by the Rust crate this will result in a jtd::SerdeSchema with a type that is Some("") and the enum set. If I then try to convert the jtd::SerdeSchema to a jtd::Schema this will fail since this is not allowed.

fn main() {
    let data = r#"{
      "definitions": null,
      "metadata": null,
      "nullable": false,
      "ref": null,
      "type": "",
      "enum": [
        "foo",
        "bar"
      ],
      "elements": null,
      "properties": null,
      "optionalProperties": null,
      "additionalProperties": false,
      "values": null,
      "discriminator": "",
      "mapping": null
    }"#;

    let s: jtd::SerdeSchema = serde_json::from_str(data).unwrap();
    let r = jtd::Schema::from_serde_schema(s);
    let e = r.err().unwrap();

    println!("{:?}: {}", e, e);
}

InvalidForm: invalid combination of keywords in schema

This would be solved by merging #5 but I wanted to add this issue to show a use case and a reason to why we would want to use omitempty.

This applies for at least Type, Discriminators and AdditionalProperties.

(The last one is a bit weird though, that should probably be fixed on the Rust side by ignoring false instead of using Optional<bool> but that's off topic for this issue)