排序 ORDER BY NULL 其中 NULL 被转移导致查询错误

[zxr615]

1. What version of Go and system type/arch are you using?

go version go1.17.2 linux/amd64

2. What version of GoFrame are you using?

github.com/gogf/gf v1.16.6

3. Can this issue be re-produced with the latest release?

是

4. What did you do?

-- gdb 生成出来的sql NULL 被转义了，导致查询认为 NULL 是一个字段，从而导致 sql 出错
SELECT a, COUNT(*) FROM bar GROUP BY a ORDER BY `NULL`;

5. What did you expect to see?

期待 NULL 值不被转义

SELECT a, COUNT(*) FROM bar GROUP BY a ORDER BY NULL;

6. What did you see instead?

#1499