[BUGFIX] Filter null collections (kitodo#1461)

Classes/Common/Solr/SolrSearch.php

@@ -781,6 +781,9 @@ private function getCollectionFilterQuery(string $query) : string
     {
         $collectionsQueryString = '';
         $virtualCollectionsQueryString = '';
+
+        $this->filterCollections();
+
         foreach ($this->collections as $collection) {
             // check for virtual collections query string
             if ($collection->getIndexSearch()) {
@@ -808,6 +811,18 @@ private function getCollectionFilterQuery(string $query) : string
         return implode(' OR ', array_filter([$collectionsQueryString, $virtualCollectionsQueryString]));
     }
 
+    /**
+     * Filter collections to avoid null values.
+     *
+     * @return void
+     */
+    private function filterCollections(): void
+    {
+        if (is_array($this->collections)) {
+            array_filter($this->collections, fn($value) => $value !== null);
+        }
+    }
+
     /**
      * Get sort order of the results as given or by title as default.
      *