Update RusiMessagingTransport.cs

Fixed pubsub name default value to empty string when null

src/Messaging/NBB.Messaging.Rusi/RusiMessagingTransport.cs

@@ -76,7 +76,7 @@ public async Task<IDisposable> SubscribeAsync(string topic, Func<TransportReceiv
 
             var subRequest = new SubscriptionRequest()
             {
-                PubsubName = _options.Value.PubsubName,
+                PubsubName = _options.Value.PubsubName ?? string.Empty,
                 Topic = topic,
                 Options = subscriptionOptions
             };