reverted special null check syntax change

com.unity.perception/Runtime/Randomization/Randomizers/SampleRandomizers/Randomizers/BackgroundObjectPlacementRandomizer.cs

         /// </summary>
         protected override void OnIterationStart()
         {
-            m_SpawnedObjects ??= new List<GameObject>();
+            if (m_SpawnedObjects == null)
+                m_SpawnedObjects = new List<GameObject>();
 
             for (var i = 0; i < layerCount; i++)
             {