Bug 26915: Fix FinePaymentAutoPopup behaviour

Bug 26915: Fix FinePaymentAutoPopup behaviour

The behaviour of the syspref FinePaymentAutoPopup is broken, the popup
explodes with
    Can't call method "unblessed" on an undefined value at /home/vagrant/kohaclone/members/printfeercpt.pl line 64

It has been broken by
  commit a2449a81be55aa5b3710ca2c6df7884513f89540
  Bug 23051: (follow-up) Add renewal feedback and move code to subroutines and test
which modified the return value of Koha::Account->pay (from $payment_id
to a hashref)

Test plan:
- To test activate FinePaymentAutoPopup
- Create a manual fee
- Write it off using the write off button on the account line (not below table)
=> Without the patch you get the error
=> With the patch applied the popup is correctly displayed

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>