Bug 23081: set default to 0 for items.issues and deleteditems.issues

Bug 23081: set default to 0 for items.issues and deleteditems.issues

To test:
- save and run this sql query in reports: select sum(if(issues is null,1,0)),sum(if(issues=0,1,0)) from items
- you should see a lot of nulls and no zeros
- apply patch
- updatedatabase
- re-run your query and see that your nulls have changed to zeros
- create a new item
- rerun your query and see your new item is counted in the zeros, not the nulls

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