Bug 19694: Force scalar context for output_pref called with billingdate

If no string is passed to output_pref, it needs to be called in scalar
context (to avoid a shift in the hash elements).
Here we have billingdate that is not defined yet (NULL)

Test plan:
- Search for an existing invoice
- Show details
- Changing shipping cost
- Save
- Verify the new amount is shown

Signed-off-by: Jon Knight <J.P.Knight@lboro.ac.uk>

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>

acqui/invoice.pl

@@ -84,8 +84,8 @@ elsif ( $op && $op eq 'mod' ) {
     ModInvoice(
         invoiceid             => $invoiceid,
         invoicenumber         => $invoicenumber,
-        shipmentdate          => output_pref( { str => scalar $input->param('shipmentdate'), dateformat => 'iso', dateonly => 1 } ),
-        billingdate           => output_pref( { str => scalar $input->param('billingdate'),  dateformat => 'iso', dateonly => 1 } ),
+        shipmentdate          => scalar output_pref( { str => scalar $input->param('shipmentdate'), dateformat => 'iso', dateonly => 1 } ),
+        billingdate           => scalar output_pref( { str => scalar $input->param('billingdate'),  dateformat => 'iso', dateonly => 1 } ),
         shipmentcost          => $shipmentcost,
         shipmentcost_budgetid => $shipment_budget_id
     );