Bug 19694: Force scalar context for output_pref called with billingdate

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>