Bug 29759: (follow-up) Fix TemplateToolkit.t

Bug 29759: (follow-up) Fix TemplateToolkit.t

This patch makes the tests set debit_id to undef instead of random data.
This way tests don't break when $article_request->cancel is invoked and
a refund is tried.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/Letters/TemplateToolkit.t
=> FAIL: Boo, tests fail, random garbage is not a debit
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests pass!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>