Fix nullable deprecation

src/DelegatingContainer.php

@@ -34,7 +34,7 @@ class DelegatingContainer implements ContainerInterface
 
     /**
      */
-    public function __construct(ServiceProviderInterface $provider, PsrContainerInterface $parent = null)
+    public function __construct(ServiceProviderInterface $provider, ?PsrContainerInterface $parent = null)
     {
         $this->provider = $provider;
         $this->parent = $parent;

src/Exception/NotFoundException.php

@@ -10,14 +10,14 @@
 class NotFoundException extends ContainerException implements NotFoundExceptionInterface
 {
     /**
-     * @param string         $message  The error message.
-     * @param int            $code     The error code.
-     * @param Throwable|null $previous The inner error, if any.
+     * @param string     $message  The error message.
+     * @param int        $code     The error code.
+     * @param ?Throwable $previous The inner error, if any.
      */
     public function __construct(
         string $message = '',
         int $code = 0,
-        Throwable $previous = null
+        ?Throwable $previous = null
     ) {
         parent::__construct($message, $code, $previous);
     }

src/ProxyContainer.php

@@ -24,10 +24,10 @@ class ProxyContainer implements BaseContainerInterface
     protected $innerContainer;
 
     /**
-     * @param BaseContainerInterface|null $innerContainer The inner container, if any.
-     *                                                    May also be set later with {@see setInnerContainer()}.
+     * @param ?BaseContainerInterface $innerContainer The inner container, if any.
+     *                                                May also be set later with {@see setInnerContainer()}.
      */
-    public function __construct(BaseContainerInterface $innerContainer = null)
+    public function __construct(?BaseContainerInterface $innerContainer = null)
     {
         $this->innerContainer = $innerContainer;
     }