protected void fillNullValue(TimeValuePair v, TimeValuePair c) {
    // do nothing for non-aligned time series
  }
this method only take effect for aligned time series, so the override version.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/read/reader/common/PriorityMergeReader.java