code: func isSingleColIdxNullRange(idx *Index, ran *ranger.Range) bool {
  	if len(idx.Info.Columns) > 1 {
  		return false
  	}
  	l, h := ran.LowVal[0], ran.HighVal[0]
  	if l.IsNull() && h.IsNull() {
  		return true
  	}
  	return false
  }
docs: // isSingleColIdxNullRange checks if a range is [NULL, NULL] on a single-column index.