Bug 13675: Do not set aqorders.budget_id to NULL

Bug 13675: Do not set aqorders.budget_id to NULL

C4::Acquisition::ModReceiveOrder updates the aqorders with
budget_id=NULL if no budget_id given in parameter.
Actually the same budget_id should be used.
In tests (especially t/db_dependent/Acquisition/TransferOrder.t),
ModReceiveOrder is not called with a budget_id param and set to NULL the
budget_id value.

test plan:
  prove t/db_dependent/Acquisition/TransferOrder.t
should return green

Note that this bug should not appear using the interface.

Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
The fix makes sense, and running
 $ prove t/db_dependent/Acq*
returns all green. koha-qa.pl also likes it.

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>