Reflect that $payload can be either string or null.

Author: Jan-Marten de Boer

src/WebhookSignature.php

@@ -14,7 +14,7 @@ public function __construct($secret)
 
     /**
      * @param string $signature
-     * @param string $payload
+     * @param ?string $payload
      * @return bool
      */
     public function validate($signature, $payload)