protected boolean isEmpty(int columnIndex) {
    return inputTsBlocks[columnIndex] == null
        || inputTsBlocks[columnIndex].getPositionCount() == inputIndex[columnIndex];
  }
If the tsBlock of columnIndex is null or has no more data in the tsBlock, return true; else
return false.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/process/join/InnerTimeJoinOperator.java