Make optional properties optional, not just nullable

[schicks]

Adding a nullable or defaulted field to a record creates a sane avro schema, but still requires the field to be provided (though it can be null or undefined).

const Person = Record('person', {name: Field({type: String(), default: 'odysseus'})

const nobody: AsTypescript<typeof Person> = {} // type error! {} has no property 'name'