Header doesnt encode and decode the same way

[hejfelix]

Reproduce:

import sttp.apispec.openapi.Header
import sttp.apispec.openapi.circe.*
import io.circe.syntax.*
val header = Header()

val actual = header.asJson.as[Header]
println(actual)

outputs:

Left(DecodingFailure at .examples: Got value 'null' with wrong type, expecting object)

the end result is that I cannot parse json or yaml produced with this library.