Bug 27580: Handle NULL values correctly in Koha::Items->filter_by_visible_in_opac

Bug 27580: Handle NULL values correctly in Koha::Items->filter_by_visible_in_opac

This patch fixes the internally used query so it doesn't consider NULL
values as IN the set.

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Items.t
=> FAIL: Tests fail :-/
3. Apply this patch
4. Repeat 2
=> SUCCESS: tests pass!
5. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>