Bug 29561: Remove blank facets

Bug 29561: Remove blank facets

This patch strips the blank facets from the ES aggregations, this seems
to only affect language fields currently, but could affect any facetable field
that can contain blank values

To test:
1 - Have Koha running Elasticsearch with standard test data,
    or add some records with blank values in the 008/35-37
2 - Search for 'a'
3 - Look at the language facets, they should appear normal
4 - Browse to:
    http://localhost:8081/cgi-bin/koha/catalogue/search.pl?idx=&q=a&sort_by=relevance_dsc&count=20&limit=ln:
5 - Note there is now an 'x' under the Languages facet - to remove a limit you cannot see
6 - Apply patch
7 - Reload
8 - Note the x is not there
9 - Inspect HTML, confirm no blank link under language facets

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>