[5.8] Don't cycle remember token on logout if not set

[andreasnij]

The remember token was cycled even though the remember functionality never was used and the token set. In the database it looked like all the users had used the functionality, which was confusing. This change stops that and only cycles the token if it is set.

[taylorotwell]

I still see no actual point in changing this.

[andreasnij]

The point and benefit to the Laravel users is that it makes the user database data less confusing and lets you see which users has used the "remember me" functionality. We have applications that doesn't have a "remember me" checkbox, and yet our database is full of remember tokens.