Bug 18434 - Followup - same changes for sort and facet fields

Bug 18434 - Followup - same changes for sort and facet fields

To test:
1 - Index some stuff with multiple fields defined for sorting
  i.e. Authorites - make heading sortable - default is 110a and 111a for
  heading - a record with 111a empty will make the sort field empty
2 - view the record:
curl http://localhost:9200/koha_kohadev_authorities/data/30?pretty=true
3 - Note the blank field
4 - Apply patch
5 - Reindex
6 - Fields should be correctly populated

Unit tests to follow (once I have the originals working for all)

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>