Bug 36081: (QA follow-up) Resolve IsNotDebit exception in ArticleRequests.t

Bug 36081: (QA follow-up) Resolve IsNotDebit exception in ArticleRequests.t

Resolve (when running ArticleRequests.t):
Exception 'Koha::Exceptions::Account::IsNotDebit' thrown 'Account line 326 is not a debit'

This occurs after switching from ArticleRequest->new to TestBuilder.
TestBuilder creates an account line that has a credit_type_code and
a debit_type_code. (This could be fixed further somewhere else.)
For now, just setting debit_id to NULL.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>