使用窗函数lead和lag时，default参数必须为null

[italyfiori]

Describe the bug
使用窗函数lead和lag时，默认值的参数如果不为NULL时，语句报错

To Reproduce
Steps to reproduce the behavior:

1. create table

CREATE TABLE `table_test` (
  `active_day` date NOT NULL COMMENT "日期",
  `cuid` varchar(64) NOT NULL COMMENT "操作",
  `time` datetime NOT NULL COMMENT "上传时间",
  `operation` varchar(64) NOT NULL COMMENT "操作"
) ENGINE=OLAP
UNIQUE KEY(`active_day`, `cuid`, `time`, `operation`)
DISTRIBUTED BY HASH(`active_day`) BUCKETS 10
PROPERTIES (
"storage_type" = "COLUMN"
); 

2. insert data
   省略
3. 查询语句中使用窗函数时提示错误

SELECT cuid,
       operation,
       time,
       lag(operation, 1, '') over (partition BY cuid ORDER BY cuid, time) as last_op
FROM table_test limit 10;

ERROR 1064 (HY000): Type = VARCHAR() can't not convert to VARCHAR()

Expected behavior
SQL正常执行

Additional context
lag或lead的第3个参数改为null时，sql可以正常执行

SELECT cuid,
       operation,
       time,
       lag(operation, 1, null) over (partition BY cuid ORDER BY cuid, time) as last_op
FROM table_test limit 10;

[EmmyMiao87]

#1846