fix create default sensor

com.unity.ml-agents/Runtime/Sensors/RayPerceptionSensor.cs

         /// </summary>
         public virtual int GetObservationSizePerRay()
         {
-            return RayPerceptionInput.DetectableTags.Count + 2;
+            return (RayPerceptionInput.DetectableTags?.Count ?? 0) + 2;
         }
 
         /// <summary>
         public int GetNumberOfRays()
         {
-            return RayPerceptionInput.Angles.Count;
+            return RayPerceptionInput.Angles?.Count ?? 0;
         }
 
         void SetNumObservations(int observationsSizePerRay, int numRays)