Bug 18923: Warning in Biblio::GetCOinSBiblio

Bug 18923: Warning in Biblio::GetCOinSBiblio

Use of uninitialized value in concatenation (.) or string at C4/Biblio.pm line 1456.
Note: In current master this is now line 1370 (Oct 9, 2017).

Test plan:
Enable COinSinOPACResults.
Select a record with leader pos6==a and pos7==a. This triggers genre to be
journalArticle and titletype to be a.
Without this patch, do an opac search that includes this record.
Check the log. You should see the warning.
Apply this patch, search again and check the log. The warning should not be
repeated again.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>