Fix nullable type hint for previous throwable in DecryptException constructor

Author: MayMeow

src/Exceptions/DecryptException.php

@@ -6,7 +6,7 @@
 
 class DecryptException extends \Exception
 {
-    public function __construct(string $message = "Cannot decrypt text", int $code = 0, Throwable $previous = null)
+    public function __construct(string $message = "Cannot decrypt text", int $code = 0, ?Throwable $previous = null)
     {
         parent::__construct($message, $code, $previous);
     }