[scala-akka-http-server] model default value when field is not required

[jsoizo]

fix: #12067

before: non-required fields have no default value

final case class Pet (
  id: Option[Long] = None,
  category: Option[Category] = None,
  name: String,
  photoUrls: Seq[String],
  tags: Option[Seq[Tag]] = None,
  status: Option[String] = None
)

val pet = Pet(name = "Dug", photoUrls = Nil)

after this PR: non-required fields have default value None.

final case class Pet (
  id: Option[Long],
  category: Option[Category],
  name: String,
  photoUrls: Seq[String],
  tags: Option[Seq[Tag]],
  status: Option[String]
)

val pet = Pet(id = None, category = None, name = "Dug", photoUrls = Nil, tags = None, status = None)

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[wing328]

FYI. I've updated the PR description (swapped the sample outputs)