Handling PATCH requests with Akka HTTP and circe for nullable fields

Is there a common approach to handle PATCH requests in REST API using circe library? By default, circe does not allow decoding partial JSON with only a part of the fields specified, i.e. it requires all fields to be set. You could use a withDefaults config, but it will be impossible to know if the field you received is null or just not specified. Here is a simplified sample of the possible solution. It uses Left[Unit] as a value to handle cases when the field is not specified at all:

# possible payloads
{
  "firstName": "Foo",
  "lastName": "Bar"
}
{
  "firstName": "Foo"
}
{
  "firstName": null
}

import de.heikoseeberger.akkahttpcirce.FailFastCirceSupport._
import io.circe.generic.auto._
import io.circe.{Decoder, HCursor}

case class User(firstName: Option[String], lastName: String)

// In PATCH request only 1 field can be specified. The rest could be omitted. Left represents `not specified`
case class PatchUserRequest(firstName: Either[Unit, Option[String]], lastName: Either[Unit, String])
object PatchUserRequest {
  implicit val decode: Decoder[PatchUserRequest] = new Decoder[PatchUserRequest] {
    final def apply(c: HCursor): Decoder.Result[PatchUserRequest] =
      for {
        // Here we handle `no field specified` error cases as Left[Unit]
        foo <- c.downField("firstName").as[Option[String]] match {
          case Left(noFieldSpecified) => Right(Left(()))
          case Right(result) => Right(Right(result))
        }
        bar <- c.downField("lastName").as[String] match {
          case Left(noFieldSpecified) => Right(Left(()))
          case Right(result) => Right(Right(result))
        }
      } yield PatchUserRequest(foo, bar)
  }
}

object Apis extends Directives {
 var user = User("Foo", "Bar")

 val create = path("user")(post(entity(as[User])(newUser => user = newUser)))
 val patch = path("user")(patch(entity(as[PatchUserRequest])(patchRequest => patch(patchRequest))))


// If field is specified - update the record, ignore otherwise
def patch(request: PatchUserRequest) {
  request.firstName.foreach(newFirstName => user = user.copy(firstName = newFirstName)
  request.lastName.foreach(newlastName => user = user.copy(lastName = newlastName)
}

Is there a better way to handle PATCH requests (with nullable fields) instead of writing custom codec that falls back to no value if field is not specified in the JSON payload? Thanks