docs: @Override
      public void serialize(Callback value, JsonGenerator jgen, SerializerProvider provider) throws IOException {
          if (StringUtils.isBlank(value.getRef())) {
              jgen.writeStartObject();
              for (Entry<String, PathItem> entry : value.entrySet()) {
                  jgen.writeObjectField(entry.getKey(), entry.getValue());
              }
              jgen.writeEndObject();
  
          } else {
              jgen.writeStartObject();
              jgen.writeStringField("$ref", value.getRef());
              jgen.writeEndObject();
          }
      }