Bug 22435: (follow-up) Set CREATE offset for credits to positive

Bug 22435: (follow-up) Set CREATE offset for credits to positive

The history view of an accountline shows increases and decreases in the
amount outstanding from the perspective of the credit/debit line you are
inspecting.

Apply creates a negative offset amount which acts to 'reduce' both the
outstanding 'debt' for the debit line and the outstanding 'credit' for
the credit line.  Starting with a negative offset for the credit thus
makes this nonsenical.

Void creates positive offset amounts which act to 'restore' both the
outstanding 'debt' for the debit line and the outstanding 'credit' for
the credit line.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>