Bug 36879: Spurious warning in QueryBuilder

Test plan:

With ElasticSearch enabled,
* Perform a search using the default  sort order
  (i.e. 'relevance').
* Verify that no warnings are generated in
  plack-intranet-error.log

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>

Koha/SearchEngine/Elasticsearch/QueryBuilder.pm

@@ -720,10 +720,12 @@ sub _convert_sort_fields {
     # Convert the fields and orders, drop anything we don't know about.
     grep { $_->{field} } map {
         my ( $f, $d ) = /(.+)_(.+)/;
-        {
+        defined $f && defined $d
+            ? {
             field     => $sort_field_convert{$f},
             direction => $sort_order_convert{$d}
-        }
+            }
+            : { field => undef };
     } @sort_by;
 }