Bug 31244: Fix 500 if logout and not logged in

Bug 31244: Fix 500 if logout and not logged in

Can't use an undefined value as a HASH reference at /kohadevbox/koha/C4/Auth.pm line 985
        C4::Auth::checkauth('CGI=HASH(0x5603b7dc4300)', 0, 'HASH(0x5603b2633238)', 'intranet', undef, 'intranet-main.tt') called at /kohadevbox/koha/C4/Auth.pm line 186
        C4::Auth::get_template_and_user('HASH(0x5603b7b83d08)') called at /kohadevbox/koha/mainpage.pl line 40

Test plan:
Open a private window
Hit /cgi-bin/koha/mainpage.pl?logout.x=1

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>