Bug 31908: Resolve second login with another userid

Bug 31908: Resolve second login with another userid

Somewhere the line undef $userid got removed.
We need it to resolve the second login situation.

Test plan:
Login in staff with user missing privileges.
On the login form login again with another staff user.
Note that you do no longer crash.

Run t/db../Auth.t
Run t/db../Koha/Auth/TwoFactorAuth.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: David Cook <dcook@prosentient.com.au>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>