fix: exception due to illegal access to item in an empty map (#747)

Author: shawngustaw

packages/aws-appsync-subscription-link/src/realtime-subscription-handshake-link.ts

@@ -347,7 +347,7 @@ export class AppSyncRealTimeSubscriptionHandshakeLink extends ApolloLink {
     const {
       subscriptionFailedCallback,
       subscriptionReadyCallback
-    } = this.subscriptionObserverMap.get(subscriptionId);
+    } = this.subscriptionObserverMap.get(subscriptionId) || {};
 
     // This must be done before sending the message in order to be listening immediately
     this.subscriptionObserverMap.set(subscriptionId, {