subscribe to non empty topics

src/Artemis.Plugins.Mqtt/MqttConnector.cs

@@ -79,7 +79,7 @@ public async Task Start(MqttConnectionSettings settings, IEnumerable<string> top
         await _client.StopAsync();
         await _client.StartAsync(managedClientOptions);
         await Task.WhenAll(
-            topics.Select(topic => _client.SubscribeAsync(topic))
+            topics.Where(t => !string.IsNullOrWhiteSpace(t)).Select(topic => _client.SubscribeAsync(topic))
         );
         await _client.SubscribeAsync("#");
     }