Bug 21613: (QA follow-up) Resolve warning on $ENV{_}

Bug 21613: (QA follow-up) Resolve warning on $ENV{_}

Some shells may not pass the program name in underscore, and cron also
does not like it here:
Use of uninitialized value $ENV{"_"} in pattern match (m//) at Koha/Database.pm line 79.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>