Bug 24669: Show "Unlimited" values correctly in smart-rules.pl

Bug 24669: Show "Unlimited" values correctly in smart-rules.pl

The rules "Holds allowed (total)", "Holds allowed (daily)", "Holds per
record (count)" were not set empty when in rule editing. Also, the
text "Unlimited" was not shown for rule "Holds per record (count)"
even though it should have been since empty value means unlimited
holds per record.

To test:
1. Apply patch
2. Edit the rules "Holds allowed (total)", "Holds allowed (daily)", "Holds per
record (count)" to be empty and save
3. Make sure all of the above rules now say Unlimited
4. Edit the rules again and make sure the above rules are now in the
rule editing empty.

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