Fix passing null to parameter of type string

Passing `null` to internal PHP functions, with argument type of string
is deprecated in PHP 8.1.

This can be easily fixed by supplying empty strings instead.

Author: Jan-Marten de Boer

src/HttpClient/Plugin/RequestSignature.php

@@ -84,6 +84,6 @@ private function normalizeParameters(array $params)
     {
         uksort($params, 'strcmp');
 
-        return http_build_query($params, null, '&', PHP_QUERY_RFC3986);
+        return http_build_query($params, '', '&', PHP_QUERY_RFC3986);
     }
 }

src/WebhookSignature.php

@@ -19,7 +19,7 @@ public function __construct($secret)
      */
     public function validate($signature, $payload)
     {
-        $payloadSignature = 'sha1='.hash_hmac('sha1', $payload, $this->secret);
+        $payloadSignature = 'sha1='.hash_hmac('sha1', $payload ?? '', $this->secret);
 
         return hash_equals($payloadSignature, (string) $signature);
     }