presto-main/src/main/java/io/prestosql/operator/EnforceSingleRowOperator.java/EnforceSingleRowOperator/finish
class EnforceSingleRowOperator:    @Override
    public void finish()
    {
        if (!finishing && page == null) {
            this.page = SINGLE_NULL_VALUE_PAGE;
        }
        finishing = true;
    }