Fix decoding an optional Document.DNull field

[zetashift]

Fixes: #697

Test was made from kubukoz's reproduction example:

//> using scala "3.2.1"
//> using lib "com.disneystreaming.smithy4s::smithy4s-core:0.17.2"
import smithy4s.schema.Schema
import smithy4s.Document

@main def demo = println {
  val optionalFieldSchema =
    Schema
      .struct[Option[String]](Schema.string.optional[Option[String]]("test", identity))(identity)

  Document
    .Decoder
    .fromSchema(optionalFieldSchema)
    .decode(Document.obj("test" -> Document.DNull))
}

[kubukoz]

issue: this is exactly what I was thinking, how dare you steal my ideas :P /s

[zetashift]

Haha, great minds think alike? 👀

[kubukoz]

Thanks for the contribution @zetashift!