Added missing return type (#189)

Author: staabm

rules/DowngradePhp73/Rector/List_/DowngradeListReferenceAssignmentRector.php

@@ -90,8 +90,9 @@ public function getNodeTypes(): array
 
     /**
      * @param Expression $node
+     * @return Node[]|null
      */
-    public function refactor(Node $node)
+    public function refactor(Node $node): ?array
     {
         if (! $node->expr instanceof Assign) {
             return null;