[Fix] If no agents are connected to an external brain, the external communicator should not try to update the agents (#399)

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

             {
                 var brainName = brain.gameObject.name;
 
+                if (current_agents[brainName].Count() == 0)
+                {
+                    continue;
+                }
                 var memorySize = rMessage.memory[brainName].Count() / current_agents[brainName].Count();
 
                 for (int i = 0; i < current_agents[brainName].Count(); i++)