Bug 19048: (bug 17829 follow-up) Fix regression in self checkout

Bug 19048: (bug 17829 follow-up) Fix regression in self checkout

$patronid is not necessarily set or does not match a valid cardnumber.
These cases must be taken into account to avoid the script to crash and
raise the following error:
Can't call method "unblessed" on an undefined value at
(...)/koha/opac/sco/sco-main.pl line 117

Test plan:
Hit sco/sco-main.pl and confirm that the error is gone with this patch
applied

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>