jumbojett#413 requestTokenExchange fixed to support PHP8.x

Changed parameter jumbojett#2 of http_build_query to empty string

src/OpenIDConnectClient.php

@@ -972,7 +972,7 @@ public function requestTokenExchange(string $subjectToken, string $subjectTokenT
         }
 
         // Convert token params to string format
-        $post_params = http_build_query($post_data, null, '&', $this->encType);
+        $post_params = http_build_query($post_data, '', '&', $this->encType);
 
         return json_decode($this->fetchURL($token_endpoint, $post_params, $headers), false);
     }