JDBC adapter should generate "CAST(NULL AS type)" rather than "NULL"

select cast(null as varchar) as a

"SELECT NULL AS `A`\n"
            + "FROM (VALUES  (0)) AS `t` (`ZERO`)"

"SELECT CAST(NULL AS VARCHAR) AS `A`\n"
            + "FROM (VALUES  (0)) AS `t` (`ZERO`)"

// code placeholder
@Test
public void testSeletNull() throws SqlParseException {
    String sql = "select cast(null as varchar) as a";
    SqlNode sqlNode = parserContext.parseStmt(sql);
    RelNode relNode = parserContext.getSqlToRelConverter().convertQuery(sqlNode, true, true).rel;
    SqlNode sqlNodeNew = toSqlNode(relNode);
    Assert.assertEquals(sqlNodeNew.toString(), "SELECT NULL AS `A`\n"
            + "FROM (VALUES  (0)) AS `t` (`ZERO`)");
}

private static SqlNode toSqlNode(RelNode root) {
  SqlDialect dialect = MysqlSqlDialect.DEFAULT;
  RelToSqlConverter converter = new RelToSqlConverter(dialect == null ? dialect : dialect);
  return converter.visitChild(0, root).asStatement();
}