JSON codec maps None to empty body, keeps content type

Currently, Codec#json(), used by TapirJsonCirce, will interpret an output type of Option[*] as designating an optional body: If the value is None, the response will override the underlying JSON codec's rendering to null and produce an empty body instead, which is no legal JSON, but still declare a content type of application/json.

import sttp.tapir._
import sttp.tapir.json.circe._
val codec = implicitly[Codec[String, Option[String], CodecFormat.Json]]
s"${codec.format.mediaType}: '${codec.encode(None)}'" // application/json: ''

This could either be helped by omitting the Content-type header upon empty body (and perhaps forcing a 204 status code?), or by removing the in-band signaling, i.e. encoding Option values according to the underlying codec and handling the optional body case elsewhere.

Discussion: https://softwaremill.community/t/why-map-none-to-empty-string-in-json-codec/381

Start with Codec#json() and its use by TapirJsonCirce, then review the linked discussion to understand the competing behavior options. Determine whether optional JSON values should encode as JSON null or be handled as an absent body, and define regression coverage for the chosen behavior, including content type handling.