code: private static ProductPartitionTreeImpl createEmptyAdGroupTree(Long adGroupId,
        BiddingStrategyConfiguration biddingStrategyConfig) {
      Preconditions.checkNotNull(adGroupId, "Null ad group ID");
      Preconditions.checkNotNull(biddingStrategyConfig, "Null bidding strategy configuration");
      ProductPartitionNode rootNode =
          new ProductPartitionNode(null, null, -1L, new ProductDimensionComparator());
      return new ProductPartitionTreeImpl(adGroupId, biddingStrategyConfig, rootNode);
    }
docs: Returns a new empty tree.
  
  @param adGroupId the ID of the ad group
  @param biddingStrategyConfig the bidding strategy configuration of the ad group