Standardize nullable type hints to 'null|Type' order

Refactors type hints and docblocks across the codebase to consistently use the 'null|Type' order for nullable types, improving code readability and alignment with modern PHP standards. No functional changes are introduced; this is a documentation and type annotation update only.

Author: huangdijia

src/PipeMessage.php

@@ -42,7 +42,7 @@ public function handle(): void
     /**
      * @template T
      * @param class-string<T> $class
-     * @return T|null
+     * @return null|T
      */
     private function get(string $class)
     {

src/Sender.php

@@ -124,7 +124,7 @@ public function setResponse(int $fd, mixed $response): void
     }
 
     /**
-     * @return Response|ServerConnection|null
+     * @return null|Response|ServerConnection
      */
     public function getResponse(int $fd): mixed
     {