UUM-138027: Fix APV breaking when cleared twice

Author: pigselated

Packages/com.unity.render-pipelines.core/Runtime/Lighting/ProbeVolume/ProbeReferenceVolume.cs

@@ -1469,11 +1469,13 @@ internal void RemovePendingScene(string sceneGUID, List<int> cellList)
             }
 
             // Remove bricks and empty cells
-            foreach (var cellIndex in cellList)
+            if (cellList != null)
             {
-                RemoveCell(cellIndex);
+                foreach (var cellIndex in cellList)
+                {
+                    RemoveCell(cellIndex);
+                }
             }
-
             ClearDebugData();
             ComputeCellGlobalInfo();
         }