create table add default null

```
> create table t1(c1 string default null);
Error: [2000] default value type mismatch
```

`default null` is needless, but it should pass, and the error is not good.

Look for SQL parser and type checking code related to CREATE TABLE and DEFAULT clause validation. The error 'default value type mismatch' suggests a type validation issue when NULL is specified. Find where default values are validated and ensure NULL is accepted for any column type. Test by creating a table with a string column default null.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.