chore: fix phpDoc for return values

src/Plugin/CookiePlugin.php

@@ -164,7 +164,7 @@ private function createCookie(RequestInterface $request, string $setCookieHeader
      *
      * @param string $part A single cookie value in format key=value
      *
-     * @return array{0:string, 1:?string}
+     * @return array{0:string, 1:string|null}
      */
     private function createValueKey(string $part): array
     {