fix(graphql): fix error is throwed 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)

packages/graphql/lib/src/links/websocket_link/websocket_client.dart

@@ -215,7 +215,8 @@ class SocketClient {
   final SocketClientConfig config;
 
   final BehaviorSubject<SocketConnectionState> _connectionStateController =
-      BehaviorSubject<SocketConnectionState>();
+      BehaviorSubject<SocketConnectionState>.seeded(
+          SocketConnectionState.notConnected);
 
   final HashMap<String, SubscriptionListener> _subscriptionInitializers =
       HashMap();