Bug 19873: Ability to search on 0 value

Bug 19873: Ability to search on 0 value

Test plan:
  - create a st-numeric index in zebra conf related to
    a numeric field
    i.e:
    yourindex 1=yourindex 4=109
  - fill a field with 0 in a biblio,
  - reindex your biblios,
  - search yourindex=0,
  - should not work,
  - apply this patch,
  - test again,
  - should work

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>