@Override
  protected void fillNullValue(TimeValuePair v, TimeValuePair c) {
    fillNullValueInAligned(v, c);
  }
In the case of aligned time seres, like d1.s1, d1.s2, d1.s3, we may first only write d1.s2,
d1.s3 (2, 3)and then flush it into one tsfile, and then an unseq record come, it contains only
d1.s1 and d1.s3 -> (10,30). So now we have [null, 2, 3] with low priority and [10, null, 30]
with high priority. we use [null, 2, 3] to fill null value in [10, null, 30], so we get [10, 2,
30], we won't use 3 to replace 30, because [10, null, 30] has higher priority

@param v TimeValuePair with high priority needs to be filled
@param c TimeValuePair with low priority is used to fill the v
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/read/reader/common/AlignedPriorityMergeReader.java