OpenAPI genarator crashes if it contains an outCodec with empty mediaType map

Can be reproduced with below snippet: ``` scala //> using dep dev.zio::zio-http:3.8.0 import zio.http.endpoint.Endpoint import zio.http.RoutePattern import zio.http.codec.StatusCodec import zio.http.endpoint.openapi.{OpenAPI, OpenAPIGen} val endpoint = Endpoint(RoutePattern.GET / "e1").outCodec(StatusCodec.Created) val openAPI: OpenAPI = OpenAPIGen.fromEndpoints(title = "Experiment", version = "1.0", endpoint) println("never reached - OpenAPI generation throws an Exception") ```