When using the goctl model command to generate Go code for PostgreSQL, the auto-increment primary key cannot be null, and passing a 0 value does not trigger auto-incrementation.

[SanJan-Loe]

Describe the bug
当使用gotcl 工具，将pgsql sql文件转化为go的model代码时，的auto_increment特征导致，生成的值为int64，而不是sql.NULLINT64，调用默认生成的Insert函数时，传入的
To Reproduce
Steps to reproduce the behavior, if applicable:

1. The code is

对应的sql代码：

 "registration_order"            BIGINT    UNIQUE    GENERATED BY DEFAULT AS IDENTITY

生成的go model代码
RegistrationOrder int64 db:"registration_order"

对应的Insert代码：
func (m *defaultUserModel) Insert(ctx context.Context, data *User) (sql.Result, error) {
publicUserAccountKey := fmt.Sprintf("%s%v", cachePublicUserAccountPrefix, data.Account)
publicUserEmailKey := fmt.Sprintf("%s%v", cachePublicUserEmailPrefix, data.Email)
publicUserExtendidKey := fmt.Sprintf("%s%v", cachePublicUserExtendidPrefix, data.Extendid)
publicUserGsucidKey := fmt.Sprintf("%s%v", cachePublicUserGsucidPrefix, data.Gsucid)
publicUserIdKey := fmt.Sprintf("%s%v", cachePublicUserIdPrefix, data.Id)
publicUserPhonenumberKey := fmt.Sprintf("%s%v", cachePublicUserPhonenumberPrefix, data.Phonenumber)
ret, err := m.ExecCtx(ctx, func(ctx context.Context, conn sqlx.SqlConn) (result sql.Result, err error) {
query := fmt.Sprintf("insert into %s (%s) values ($1, $2, $3, $4, $5, $6, $7, $8, $9, $10, $11, $12, $13, $14, $15, $16)", m.table, userRowsExpectAutoSet)
return conn.ExecCtx(ctx, query, data.Id, data.Extendid, data.Gsucid, data.Name, data.Account, data.Password, data.Email, data.Phonenumber, data.Status, data.Errorlogincount, data.Refreshtoken, data.RegistrationOrder, data.Lastlogintime, data.Createtime, data.Updatetime, data.Deletetime)
}, publicUserAccountKey, publicUserEmailKey, publicUserExtendidKey, publicUserGsucidKey, publicUserIdKey, publicUserPhonenumberKey)
return ret, err
}

2. The error is

默认重复插入registration_order为0的数据时，出现如下错误：
error in inserting user:pq: duplicate key value violates unique constraint "user_registration_order_key"

[kesonan]

1. If you want sql.NULLINT64 instead of int64, you should see the document and try it again.
2. The error log maybe cause by your constraint incorrect, pls take a check, it's not cause by goctl.