Bug 20056: Resolve uninitialized warn in cmp_sysprefs.pl

Bug 20056: Resolve uninitialized warn in cmp_sysprefs.pl

When running a compare with --upd flag, I got the following warn:
    Use of uninitialized value in addition (+) at misc/maintenance/cmp_sysprefs.pl line 125.
This is simply resolved by not returning undef but 0 in case of the Version
syspref in the sub UpdateOnePref.

Test plan:
Look at this simple change.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>