fix(openapi): allow null return type

fixes #6548

Author: soyuka

src/OpenApi/Model/PathItem.php

@@ -83,7 +83,7 @@ public function getServers(): ?array
         return $this->servers;
     }
 
-    public function getParameters(): array
+    public function getParameters(): ?array
     {
         return $this->parameters;
     }