Bug 23768: Return invalid ISBN when searching with variations

Bug 23768: Return invalid ISBN when searching with variations

To test:
1 - Enable SearchWithISBNVariations and  IntranetCatalogSearchPulldown
2 - Refresh page and click on 'Search the catalog' tab
3 - Search for 'ISBN' 0385299209
4 - Note no results
5 - Note the search says if searched 'kw,wrdl: (nb=)'
6 - Apply patch, restart all the things
7 - Repeat search
8 - Search looks correctly formed
9 - Add the isbn above to a record, confirm it is returned by search
10 - Confirm searches for valid ISBNs still work as expected

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>