Bug 25440: Fix for "uninitialized $maxsuspensiondays" in smart-rules.pl

Bug 25440: Fix for "uninitialized $maxsuspensiondays" in smart-rules.pl

This warning emitted:

Use of uninitialized value $maxsuspensiondays in string eq
at /admin/smart-rules.pl line 257.

But that not just undef-warning, there is broken logic,
these two lines are mutually contradictory and goes one-by-one:

    $maxsuspensiondays = undef if $maxsuspensiondays eq q||;
    $maxsuspensiondays = '' if $maxsuspensiondays eq q||;

Fix is simple: to make it '' if it comes undef.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>