Sharding fixes

* replaced not_null check of initialized array in VuFind\Search\Solr\Params
* eliminated warning/added comments in VuFind\Search\Solr\MultiIndexListener

module/VuFind/src/VuFind/Search/Solr/MultiIndexListener.php

@@ -124,7 +124,14 @@ class MultiIndexListener
             } else {
                 // In any other context, we should make sure our field values are
                 // all legal.
-                $shards = explode(',', implode(',', $params->get('shards')));
+
+                // Normalize array of strings containing comma-separated values to
+                // simple array of values; check if $params->get('shards') returns
+                // an array to prevent invalid argument warnings.
+                $shards = $params->get('shards');
+                $shards = explode(
+                    ',', implode(',', (is_array($shards) ? $shards : array()))
+                );
                 $fields = $this->getFields($shards);
                 $specs  = $this->getSearchSpecs($fields);
                 $backend->getQueryBuilder()->setSpecs($specs);

module/VuFind/src/VuFind/Search/Solr/Params.php

@@ -459,7 +459,7 @@ class Params extends \VuFind\Search\Base\Params
         // Shards
         $allShards = $this->getOptions()->getShards();
         $shards = $this->getSelectedShards();
-        if (is_null($shards)) {
+        if (empty($shards)) {
             $shards = array_keys($allShards);
         }