Bug 35792: Use of uninitialized value $sub6

Bug 35792: Use of uninitialized value $sub6

Test plan:

1. Apply patch
2. Rerun and make sure the error "Use of uninitialized value $sub6 in pattern match (m//) at /usr/share/koha/lib/Koha/SearchEngine/Elastricsearch.pm" is no longer present.

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Initial testing done, it works well, needs an additional sign off from an external party

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>