protected DataFile toDataFile() {
      if (currentRows > 0) {
        return dataWriter.toDataFile();
      } else {
        return null;
      }
    }
a writer holder which require records had been sorted before write. The holder will hold only
one writer in open, and will throw an IllegalStateException exception if TaskWriter request a
data writer via a write key which has been already closed.
amoro-format-iceberg/src/main/java/org/apache/amoro/io/writer/BaseTaskWriter.java