there's a common pattern where request fields may need to differentiate
between an unset value and a value set to the type's zero value. this is
commonly used to apply a different default value, e.g. stream, or to
omit a field entirely, e.g. think.
similar to sql.Null[T], types.Null[T] simplifies this by providing
utilities to quickly and easily apply this pattern to any type using
generics.
