fix bug when no messages existed for queried entity

encompass-cs/Collections/TypedMessageStore.cs

@@ -90,7 +90,7 @@ namespace Encompass
 
         public IEnumerable<TMessage> WithEntity(int entityID)
         {
-            return entityToMessage[entityID];
+            return entityToMessage.ContainsKey(entityID) ? entityToMessage[entityID] : System.Linq.Enumerable.Empty<TMessage>();
         }
 
         public override void Clear()

test/EngineTest.cs

@@ -483,6 +483,20 @@ namespace Tests
             entityMessageResults.Should().ContainEquivalentOf(new EntityMessage(entity, 5));
         }
 
+        [Test]
+        public void NoMessagesWithEntity()
+        {
+            var worldBuilder = new WorldBuilder();
+
+            var entity = worldBuilder.CreateEntity();
+            worldBuilder.AddEngine(new EntityMessageReceiverEngine(entity));
+
+            var world = worldBuilder.Build();
+            world.Update(0.01);
+
+            entityMessageResults.Should().BeEmpty();
+        }
+
         class SomeComponentTestEngine : Engine
         {
             public override void Update(double dt)