Fix unset driver in similar items channel provider.

module/VuFind/src/VuFind/ChannelProvider/SimilarItems.php

@@ -162,7 +162,10 @@ class SimilarItems extends AbstractChannelProvider
         }
         // If the search results did not include the object we were looking for,
         // we need to fetch it from the search service:
-        if (empty($channels) && is_object($driver) && $channelToken !== null) {
+        if (empty($channels)
+            && is_object($driver ?? null)
+            && $channelToken !== null
+        ) {
             $driver = $this->searchService->retrieve(
                 $driver->getSourceIdentifier(), $channelToken
             )->first();