message: Handle null content range.
diff: @@ -577,7 +577,7 @@ export namespace IModelTileIO {
  readStatus: TileIO.ReadStatus.Success,
  isLeaf,
  sizeMultiplier,
  - contentRange,
  + contentRange: contentRange.isNull ? undefined : contentRange,
  renderGraphic: tileGraphic,
  };
  }
  