Bug 30781: Fix warning in GetPreparedLetter

Bug 30781: Fix warning in GetPreparedLetter

Test by running :
  prove t/db_dependent/Letters.t
Check you dont see warning :
  Use of uninitialized value $val in substitution iterator

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 34b84b68e509c9172b850dad4dbc5b7e9f518225)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 0783b96359e392b95538a7aca78f263ae178e1ca)
Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>