types: make optional fields optional
types: make optional fields optional

Closes GH-8.

Reviewed-by: Christian Murphy <87443399f33738b7ca84788ff25901f64eee28bb@gmail.com>
Reviewed-by: Junyoung Choi <abf60c2f76ee4ce1eaf46104ab03062cde6edcd5@gmail.com>
Reviewed-by: Titus Wormer <5a19dbfdb793ef6d212461b3c03b23ad19ae3ed1@gmail.com>
wooorm/stringify-entities