Fixed issue where UEA would throw an exception if nothing is listening for message.

Author: EricFreeman

src/UnityEventAggregator/EventAggregator.cs

@@ -65,6 +65,7 @@ public static void UnRegister(object obj, Type listener)
         /// <param name="message"></param>
         public static void SendMessage<T>(T message)
         {
+            if (!_cache.ContainsKey(message.GetType())) return;
             _cache[message.GetType()].Each(x => ((IListener<T>)x).Handle(message));
         }