Bug 24443: Consider NULL as 0 for issues in items search

Bug 24443: Consider NULL as 0 for issues in items search

In items search, we can filter by items issues count, data coming from items.issues.
Most of the time, for an item with no issues this column contains NULL.
This enhancement proposes to consider NULL as 0 to allow searching items with no issues.

Test plan:
1) Use SQL to count items with items.issues = NULL :
   select count(*) from items where issues is null;
2) Go to items search
3) Perform search with filter "Checkout count" = 0
4) Check you get the same number of results as SQL query

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