fix: make == override parameters non-nullable

Author: kamilkarp

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

@@ -270,7 +270,7 @@ class SubscriptionData extends GraphQLSocketMessage {
   int get hashCode => toJson().hashCode;
 
   @override
-  bool operator ==(dynamic other) =>
+  bool operator ==(Object other) =>
       other is SubscriptionData && jsonEncode(other) == jsonEncode(this);
 }
 
@@ -295,7 +295,7 @@ class SubscriptionNext extends GraphQLSocketMessage {
   int get hashCode => toJson().hashCode;
 
   @override
-  bool operator ==(dynamic other) =>
+  bool operator ==(Object other) =>
       other is SubscriptionNext && jsonEncode(other) == jsonEncode(this);
 }