@Override
    public boolean isSingleRow() {
      if (!hashMapResult.hasRows()) {
        return (dummyRow != null);
      }
      return hashMapResult.isSingleRow();
    }
Sometimes, when container is empty in multi-table mapjoin, we need to add a dummy row.
This container does not normally support adding rows; this is for the dummy row.
ql/src/java/org/apache/hadoop/hive/ql/exec/persistence/MapJoinBytesTableContainer.java