Bug 30781: Add unit test for substitute with undefined value

Bug 30781: Add unit test for substitute with undefined value

Add unit test that generates warn.
This may be usefull as no regression test.

Run prove t/db_dependent/Letters.t
You see warn :
t/db_dependent/Letters.t .. 1/84 Use of uninitialized value $val in substitution iterator at /kohadevbox/koha/C4/Letters.pm line 607.

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 2e0243c378158755660d7bc95e15ad32dd6d0ebf)

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