fix attributes value is covered to "Optional()" on iOS

[RyoheiTomiyama]

In BonsoirDiscoveryEvent, event.service.attributes value is covered to "Optional"

_bonsoirDiscovery?.eventStream?.listen(_onEventOccurred)

void _onEventOccurred(BonsoirDiscoveryEvent event) async {
    if (event.service == null || !event.isServiceResolved) {
      return;
    }
    print(event.service);

    ....
}

Actual

value in attribute is include "Optional"

flutter:  {"service.name":"iPhone 11 Pro","service.type":"_http._tcp.","service.port":4000,"service.attributes":{"localizedModel":"Optional(\"iPhone\")","model":"Optional(\"iPhone\")"},"service.ip":"127.0.0.1"}

Expected

should not be include "Optional"

flutter: {"service.name":"iPhone 11 Pro","service.type":"_http._tcp.","service.port":4000,"service.attributes":{"localizedModel":"iPhone","model":"iPhone"},"service.ip":"127.0.0.1"}

[Skyost]

Thanks a lot !