Bug 28368: Add fallback language for anonymous payment receipts

This adds a fallback for when a payment is made anonymously to allow
printing a receipt in the staff members current session language.

Test plan
1. Set `EnablePointOfSale` and `UseCashRegisters`
2. Add new cash register`
3. New debit type that 'can be sold'
4. Make a sale (via Point of Sale) using the above debit type.
5. Attempt to 'print receipt' from the subsequent prompt
6. Not the ISE: Can't call method "lang"
7. Apply patch
8. Attempt to 'print receipt' again, this time it should work.

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>

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

pos/printreceipt.pl

@@ -51,8 +51,9 @@ output_and_exit_if_error(
     }
 ) if $patron;    # Payment could have been anonymous
 
+my $lang = $patron ? $patron->lang : $template->lang;
 my $letter = C4::Letters::getletter( 'pos', 'RECEIPT',
-    C4::Context::mybranch, 'print', $patron->lang );
+    C4::Context::mybranch, 'print', $lang );
 
 $template->param(
     letter  => $letter,