HTTP4S Generator: Skip decoders/encoders for models with no fields

[mbryzek]

Through swagger conversion we can end up w/ a model with no fields resulting in a compile error:

implicit def decodeTestAPIFooError: Decoder[FooError] = Decoder.instance { c =>
     for {
      } yield {
        FooError(
        )
      }
    }
    implicit def encodeTestAPIFooError: Encoder[FooError] = Encoder.instance { t =>
      Json.fromFields(Seq(
      ).flatten)
    }

This PR fixes that by skipping generation of decoders/encoders for models with no fields. I think this is the right approach for right now as in the future there is an open issue to support models w/ no fields (and thus think better to solve for this error here).

Related issue: #627

[gheine]

Instead of if (...) None else Some(...) could do enum.values.headOption.map { _ => ...}

[gheine]

Same as above

[gheine]

LGTM