use [1,0,0..] for empty

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

         Color[] m_PerceptionColors;
         Texture2D m_PerceptionTexture;
         float[] m_CellDataBuffer;
+        protected float[] m_DefaultObservation;
 
         // Utility Constants Calculated on Init
         int m_NumCells;
             {
                 Array.Clear(m_PerceptionBuffer, 0, m_PerceptionBuffer.Length);
                 Array.Clear(m_CellDataBuffer, 0, m_CellDataBuffer.Length);
+                Array.Copy(m_DefaultObservation, m_CellDataBuffer, m_DefaultObservation.Length);
             }
             else
             {
+                m_DefaultObservation = new float[m_CellObservationSize];
             }
         }
 

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

             SensorCompressionType compression
         ) : base(name, cellScale, gridSize, detectableTags, compression)
         {
+            m_DefaultObservation[0] = 1;
-            return DetectableTags == null ? 0 : DetectableTags.Length;
+            return DetectableTags == null ? 0 : (DetectableTags.Length + 1);
         }
 
         /// <inheritdoc/>
         /// </param>
         protected override void GetObjectData(GameObject detectedObject, int tagIndex, float[] dataBuffer)
         {
-            dataBuffer[tagIndex] = 1;
+            dataBuffer[tagIndex + 1] = 1;
         }
     }
 }