WebSession: fix guest user uid in current_user

• Fixes issue with current_user uid introduced in b21c9d7 that prevented user to see record files as guest user.

Reported-by: Raquel Jimenez Encinar <raquel.jimenez.encinar@cern.ch>
Reported-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>