Bug 25683: Patron with no accountlines should have 0 outstanding (not NULL)

Bug 25683: Patron with no accountlines should have 0 outstanding (not NULL)

Test plan:
- Have a patron with nothing in accountlines
- run update_patron_categories to find patrons with fines under $5 (-fu=5)
  - Your patron is not found
  - Give your patron a manual charge of $1
  - rerun the cron, your patron is found
  - pay off your patron's fine, putting their balance at $0
  - rerun the cron, your patron is found

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>