v1.8.26 breaks in constructing Dayjs instance with the second parameter

**Describe the bug**
Calling dayjs(value, null) could breaks in the latest version v1.8.26 since null is also typeof object.

**Related code diff**
`https://github.com/iamkun/dayjs/compare/v1.8.25...v1.8.26#diff-1fdf421c05c1140f6d71444ea2b27638L36-R36`

I understand that its perrferred to pass in `undefined` rather than `null` to identify the undefined parseformat, however, the update shouldnt break the old feasible usage.