Bug 25440: Fix for "uninitialized value in string eq" in smart-rules.pl

Bug 25440: Fix for "uninitialized value in string eq" in smart-rules.pl

This warning emitted:

Use of uninitialized value in string eq at /admin/smart-rules.pl line 289.

It is solved by one line added to exclude comparison with 'on' string
when variable is 'undef'.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 5eebfcf68747b4886b23a9d4c8df37ea7d103edb)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>