Fix for #1093 (#1111)

Check in the batcher that the memories are not null before adding them to the agentInfoProto

unity-environment/Assets/ML-Agents/Scripts/Batcher.cs

             {
                 StackedVectorObservation = { info.stackedVectorObservation },
                 StoredVectorActions = { info.storedVectorActions },
-                Memories = { info.memories },
                 StoredTextActions = info.storedTextActions,
                 TextObservation = info.textObservation,
                 Reward = info.reward,
             };
+
+            if (info.memories != null)
+            {
+                agentInfoProto.Memories.Add(info.memories);
+            }
             foreach (Texture2D obs in info.visualObservations)
             {
                 agentInfoProto.VisualObservations.Add(