@Override
    public boolean test(FileTree fileTree) {
      return !fileTree.isLeaf() && fileTree.isRootEmpty();
    }
file tree can split if: - root node isn't leaf node - and no file exists in the root node

@param fileTree - file tree to split
@return true if this fileTree need split
amoro-format-iceberg/src/main/java/org/apache/amoro/optimizing/plan/MixedIcebergPartitionPlan.java