Bug 30354: Don't assign warn as letter

Bug 30354: Don't assign warn as letter

TO test:
1 - Check out an item marked for autop renewal to a patron and make it overdue
2 - Set system preference AutoRenewalNotices to follow messaging prefs
3 - set that borrower to receive both email and SMS AUTO_RENEWALS_DGST
4 - confirm your AUTO_RENEWALS_DGST does not have SMS content but does have email
5 - run the auto_renew cron
6 - item is renewed, but error from cron, and cron dies:

No circulation AUTO_RENEWALS_DGST letter transported by sms at /kohadevbox/koha/C4/Letters.pm line 583.
no letter of type 'AUTO_RENEWALS_DGST' found for borrowernumber 5. Please see sample_notices.sql at misc/cronjobs/automatic_renewals.pl line 305.
Can't use string ("1") as a HASH ref while "strict refs" in use at /kohadevbox/koha/C4/Letters.pm line 898.

7 - Apply patch
8 - Make item eligible for auto renewal agian (or checkin/checkout)
9 - Run the cron
10 - There is still 2 warn, but cron does not die:

No circulation AUTO_RENEWALS_DGST letter transported by sms at /kohadevbox/koha/C4/Letters.pm line 583.
no letter of type 'AUTO_RENEWALS_DGST' found for borrowernumber 5. Please see sample_notices.sql at misc/cronjobs/    automatic_renewals.pl line 305.

11 - Patron receives email and item is renewed

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@iki.fi>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>