How do you parse something with play json which requires null to be treated differently to unset - without eating parsing errors

ContentLicense.scala

//> using scala 2.13
//> using dep org.playframework::play-json:3.0.6

// because we were thinking through how to model a parameter in govuk-frontend
// which uses an explicit null as a sentinel for "display nothing" but where
// nothing at all is passed then it should fallback to a default.

import play.api.libs.json._
import play.api.libs.json.Reads._
import play.api.libs.functional.syntax._

import scala.language.implicitConversions

sealed trait ContentLicense
object ContentLicense {
  case object Default extends ContentLicense
  case object Disabled extends ContentLicense
  case class Custom(content: String) extends ContentLicense

  def readFromPath(path: JsPath): Reads[ContentLicense] = Reads { json =>
    path.asSingleJson(json) match {
      case _: JsUndefined               => JsSuccess(Default)
      case JsDefined(JsNull)            => JsSuccess(Disabled)
      case JsDefined(JsString(content)) => JsSuccess(Custom(content))
      case JsDefined(_) =>
        JsError(
          path,
          JsonValidationError(
            "error.invalidContentLicense",
            "expected it to be unset, a string, or null"
          )
        )
    }
  }

  implicit def unwrapOption(wrapped: Option[String]): ContentLicense = wrapped match {
    case None          => Default
    case Some(content) => Custom(content)
  }
}

case class Footer(
    name: String,
    contentLicense: ContentLicense
)
object Footer {
  implicit val reads: Reads[Footer] = (
    (__ \ "name").read[String] and
      ContentLicense.readFromPath(__ \ "contentLicense")
  )(Footer.apply _)
}

Json
  .parse("""
{
  "name": "Oscar",
  "contentLicense": "this is my license"
}
""")
  .as[Footer] // val res0: Footer = Footer(Oscar,Custom(this is my license))

Json
  .parse("""
{
  "name": "Oscar",
  "contentLicense": null
}
""")
  .as[Footer] // val res1: Footer = Footer(Oscar,Disabled)

Json
  .parse("""
{
  "name": "Oscar"
}
""")
  .as[Footer] // val res2: Footer = Footer(Oscar,Default)

Footer(
  name = "implicit conversions to help migration without breaking usages",
  contentLicense = Some("custom license")
) // val res3: Footer = Footer(implicit conversions to help migration without breaking usages,Custom(custom license))

Footer(
  name = "implicit conversion to help migration without breaking usages",
  contentLicense = None
) // val res4: Footer = Footer(implicit conversion to help migration without breaking usages,Default)

Footer(
  name = "but new way still works",
  contentLicense = ContentLicense.Custom("don't be a dick")
) // val res5: Footer = Footer(but new way still works,Custom(don't be a dick))

Json
  .parse("""
{
  "name": "Oscar",
  "contentLicense": false
}
""")
  .as[Footer] // gives error