Bug 25957: Don't push blank field values to ES document

Bug 25957: Don't push blank field values to ES document

To test:
 1 - Load the sample DB or edit a record (using advanced cataloging editor) to have a blank subfield in a field that is indexed as suggestible
 2 - For example 'author' / 100a
     100 _ _ ‡a
 3 - Index that record into Elasticsearch 5.X:
     perl misc/search_tools/rebuild_elasticsearch.pl  -v -bn 115 -b -d
 4 - Note error 'value must have length > 0'
 5 - Edit mappings to set author 100a not suggestible
 6 - perl misc/search_tools/rebuild_elasticsearch.pl  -v -bn 115 -b -d
 7 - Success
 8 - Set field to suggestible again
 9 - Apply patch
10 - perl misc/search_tools/rebuild_elasticsearch.pl  -v -bn 115 -b -d
11 - Success!

Signed-off-by: Bob Bennhoff <bbennhoff@clicweb.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>