Bug 34331: Use register from userenv if parameter not passed

Bug 34331: Use register from userenv if parameter not passed

To recreate:
1. Enable the "EnablePointofSale" syspref.
2. Set the "UseCashRegisters" syspref to use.
3. Create 3 new registers (Register A, B and C) with a float amount of $25.
4. Under POS, configure 3 new debit types (i.e. Copies, Earbuds, Friends Donation)
-- You can leave the default amounts blank, or add an amount if desired.
-- Set "Can be manually invoiced?" and "Can be sold?"
5. Under point of sale make a $5 sale of copies under Register A.
6. Under point of sale make a $10 sale of earbuds under Register B.
7. Under point of sale make a $100 sale of Friends Donation under Register C.
8. At circ/set-library.pl set your register to Register A.
9. Go back to POS and view the "Transaction history for Register A" - it shows the correct amount of $5 sold in copies.
10. Go back to circ/set-library.pl and change your register to Register B.
11. Go back to POS and view the "Transaction history for Register B" - it shows the incorrect transaction history. It is showing the transaction history for Register A.
12. Go back to circ/set-library.pl and change your register to Register C.
13. Go back to POS and view the "Transaction history for Register C" - it shows the incorrect transaction history. It is showing the transaction history for Register A.
14. Apply patch, repeat. The correct transactions should now show.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>