Parser cannot pass default value with an expression

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

version `09d12f0` (tried old version got the same behaviour)

```
package main

import (
"fmt"

"github.com/pingcap/tidb/pkg/parser"
"github.com/pingcap/tidb/pkg/parser/ast"
_ "github.com/pingcap/tidb/pkg/parser/test_driver"
)

func parse(sql string) (*ast.StmtNode, error) {
p := parser.New()

stmtNodes, _, err := p.ParseSQL(sql)
if err != nil {
return nil, err
}

return &stmtNodes[0], nil
}

func main() {
astNode, err := parse("ALTER TABLE `roles` MODIFY COLUMN `long_description` LONGTEXT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci NOT NULL DEFAULT ('');")
if err != nil {
fmt.Printf("parse error: %v\n", err.Error())
return
}
fmt.Printf("%v\n", *astNode)
}
```

### 2. What did you expect to see? (Required)

As documented by mysql, the [BLOB](https://dev.mysql.com/doc/refman/8.0/en/blob.html), [TEXT](https://dev.mysql.com/doc/refman/8.0/en/blob.html), GEOMETRY, and [JSON](https://dev.mysql.com/doc/refman/8.0/en/json.html) data types can be assigned a default value only if the value is written as an expression

### 3. What did you see instead (Required)

Run the simple script, and got error instead
```
parse error: line 1 column 131 near "'');"
```