Support optional sealed trait

[soujiro32167]

The following codec results in an error:

import vulcan.Codec
import vulcan.generic.*

sealed trait S
case class A(i: Int) extends S

val c1: Codec[S] = Codec.derive

Codec.option(c1).schema // AvroError(org.apache.avro.AvroRuntimeException: Nested union: ["null",[{"type":"record","name":"A","namespace":"avrounion","fields":[{"name":"i","type":"int"}]}]])

However, what I would like is

implicit val optionalUnion: Codec[Option[S]] = {
    import cats.implicits.catsSyntaxSemigroup
    implicit val ca: Codec[A] = Codec.derive

    Codec.union[Option[S]](alt =>
      alt[None.type] |+| alt[Some[A]]
    )
}

optionalUnion.schema 
/*
[ "null", {
  "type" : "record",
  "name" : "A",
  "namespace" : "avrounion",
  "fields" : [ {
    "name" : "i",
    "type" : "int"
  } ]
} ]
*/