Update Login.vue

[puf]

There was a breaking change in v5 of the Firebase Authentication SDK. Many developers use this code, but are then confused about errors when user.uid doesn't exist. I proposed a comment here, but an alternative is to use code that works in both cases, e.g.:

 if (user && !user.uid) user = user.user;

It's a bit cryptic, but pulls the user from within the AuthCredential if needed.

[hermuxo]

^ Thank you @puf! works