remove case
```sql
select datediff('10000-10-31', '2010-10-15')
```
because Nereids throw exception when cast '10000-10-31' to date,
but legacy planner return null on it
remove case
```sql
select datediff('10000-10-31', '2010-10-15')
```
because Nereids throw exception when cast '10000-10-31' to date,
but legacy planner return null on it