fix(graphql): fix error is throwed when toggleConnection != null & fix(graphql_flutter): upgrate connectivity_plus

[hantrungkien]

1. [graphql] fix error is throwed (althought it doesn't make any crash) when toggleConnection != null

[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: ValueStream has no value. You should check ValueStream.hasValue before accessing ValueStream.value, or use ValueStream.valueOrNull instead.
      BehaviorSubject.value (package:rxdart/src/subjects/behavior_subject.dart:146:5)
      SocketClient._listenToToggleConnection.<anonymous closure> (package:graphql/src/links/websocket_link/websocket_client.dart:264:40)
      _RootZone.runUnaryGuarded (dart:async/zone.dart:1594:10)
      _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:339:11)
      _BufferingStreamSubscription._add (dart:async/stream_impl.dart:271:7)
      _MultiStreamController.addSync (dart:async/stream_impl.dart:1101:36)
      _MultiControllerSink.add (package:rxdart/src/utils/forwarding_stream.dart:130:35)
      _TakeUntilStreamSink.onData (package:rxdart/src/transformers/take_until.dart:13:31)

2. [graphql_flutter] upgrate connectivity_plus: ^6.0.3

2 important commits:

• v6.0.2: fix(connectivity_plus): Improve iOS PathMonitorConnectivityProvider implementation and documentation fluttercommunity/plus_plugins#2763
• v6.0.3: fix(connectivity_plus): Ensure Connectivity on Android is correctly reported when lost fluttercommunity/plus_plugins#2836

[hantrungkien]

@vincenzopalazzo Please check help me. Thank you for your patience!

[vincenzopalazzo]

Thanks, waiting for the CI competition and merging it

[hantrungkien]

@vincenzopalazzo Thank you very much. Please let me know when I get a new version on pub.dev for bothgraphql andgraphql_flutter.