Bug 28144: Check if issue_id defined

Bug 28144: Check if issue_id defined

Historical OVERDUE fines may not have an issue_id - this causes
flooding of the logs when calling update fine

Trivial change, reading the code should be enough, but test plan below

To test:
1 - Create some fines without an issue_id
  - Either checkout an old version of Koha and add manually or
  - Checkout some items (backdated) to a patron, run fines.pl then
    UPDATE accountlines SET issue_id = NULL;
2 - Checkout a backdated item with fines in the circ rule to the same patron
3 - Run fines.pl
4 - Note the errors
5 - Apply patch
6 - Repeat and note no errors

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