check if group is null

Author: matthi4s

src/Query/Generator/SQL.php

@@ -248,7 +248,7 @@ private function generateFields(Query $query)
      */
     protected function generateGroup(Query $query): string
     {
-        if (!$query instanceof SelectQuery || count($query->getGroup()) === 0) {
+        if (!$query instanceof SelectQuery || $query->getGroup() === null || count($query->getGroup()) === 0) {
             return "";
         }