Checking if the Past action is null before feeding into inference (#3403)

com.unity.ml-agents/Runtime/InferenceBrain/GeneratorImpl.cs

             {
                 var info = infoSensorPair.agentInfo;
                 var pastAction = info.storedVectorActions;
-                for (var j = 0; j < actionSize; j++)
-                {
-                    tensorProxy.data[agentIndex, j] = pastAction[j];
+                if (pastAction != null){
+                    for (var j = 0; j < actionSize; j++)
+                    {
+                        tensorProxy.data[agentIndex, j] = pastAction[j];
+                    }
                 }
 
                 agentIndex++;