Add support for sorting "empty results"

Added unmapped_type to the sort options, or else ES returns an error if no results are found or the sorting option is unset on some of the results.

src/Controller/RequestDataHandler/ShopProductsSortDataHandler.php

@@ -77,7 +77,7 @@ public function retrieveData(array $requestData): array
             $orderBy = $this->channelPricingNameResolver->resolvePropertyName($channelCode);
         }
 
-        $data['sort'] = [$orderBy => ['order' => strtolower($sort)]];
+        $data['sort'] = [$orderBy => ['order' => strtolower($sort), 'unmapped_type' => 'keyword']];
 
         return $data;
     }