Bug 14445: Silences warns in letter.tt

Bug 14445: Silences warns in letter.tt

When creating a new notice, warn is triggered "Argument "" isn't numeric in numeric gt (>) at line 400". Same warn is triggered when changing Koha module option to any other module.

To test:
1) Go to Tools, then Notices & Slips
2) Click 'new notice'. Notice warn in intranet-error.log
3) Change Koha module to another module. Notice warn is triggered for every change
4) Apply patch and reload page
5) Change Koha module to another module. Notice there are no longer warns
6) Go back to Notices & Slips and click 'new notice' again. Notice there are no warns

Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>