Bug 25277: Don't specify a field in query if none passed

Bug 25277: Don't specify a field in query if none passed

== test plan ==
To test:
0 - Have Koha running with ES 6 enabled
1 - Launch the z3950 responder:
    perl misc/z3950_responder.pl --config-dir /kohadevbox/koha/etc/z3950 --debug
2 - Connect using yaz-client:
    yaz-client localhost:2100
2.1 Alternative, if you add the responder as a Z39.50 server in Koha and
  do a catalogue Z39.50 search. Don't forget to search in the keywords
  field. Not the title field.
3 - base biblios
4 - find bruce
5 - No results
6 - Stop the responder
7 - Apply patch
8 - Repeat
9 - This time you get results

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>