[PHP 8.4] Fixes for implicit nullability deprecation (#255)

Fixes all issues that emits a deprecation notice on PHP 8.4.

See:
 - [RFC](https://wiki.php.net/rfc/deprecate-implicitly-nullable-types)
 - [PHP 8.4: Implicitly nullable parameter declarations deprecated](https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated)

src/Factory/Psr17Factory.php

@@ -57,7 +57,7 @@ public function createStreamFromResource($resource): StreamInterface
         return Stream::create($resource);
     }
 
-    public function createUploadedFile(StreamInterface $stream, int $size = null, int $error = \UPLOAD_ERR_OK, string $clientFilename = null, string $clientMediaType = null): UploadedFileInterface
+    public function createUploadedFile(StreamInterface $stream, ?int $size = null, int $error = \UPLOAD_ERR_OK, ?string $clientFilename = null, ?string $clientMediaType = null): UploadedFileInterface
     {
         if (null === $size) {
             $size = $stream->getSize();

src/Response.php

@@ -39,7 +39,7 @@ class Response implements ResponseInterface
      * @param string $version Protocol version
      * @param string|null $reason Reason phrase (when empty a default will be used based on the status code)
      */
-    public function __construct(int $status = 200, array $headers = [], $body = null, string $version = '1.1', string $reason = null)
+    public function __construct(int $status = 200, array $headers = [], $body = null, string $version = '1.1', ?string $reason = null)
     {
         // If we got no body, defer initialization of the stream until Response::getBody()
         if ('' !== $body && null !== $body) {