Fixed compilation for .Net 4.0

com.unity.render-pipelines.high-definition/HDRP/Editor/Lighting/Volumetric/DensityVolumeTextureTool.cs

 
         void OnGUI()
         {
-            EditorGUILayout.LabelField(textureLabel, null);
+            EditorGUILayout.LabelField(textureLabel);
-            tileSize = EditorGUILayout.IntField(tileSizeLabel, tileSize, null);
+            tileSize = EditorGUILayout.IntField(tileSizeLabel, tileSize);
 
             bool validData = (sourceTexture != null && IsTileSizeValid());
             bool create = false;
 
             using (new EditorGUI.DisabledScope(!validData))
             {
-                create = GUILayout.Button(createLabel, null);
+                create = GUILayout.Button(createLabel);
             }
 
             if (create)