Bug 8713 - Warning message triggered by clicking Keywords to MARC mapping

Bug 8713 - Warning message triggered by clicking Keywords to MARC mapping

In the Koha Administration screen, clicking
Keywords to MARC mapping triggered an uninitialized $op warning
in the koha error logs. Added a "|| q{}" to ensure it is set.

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>