Hello, if `$key_spec = $this->config['private_key'];` on line 421 in https://github.com/amzn/amazon-pay-api-sdk-php/blob/2.2.5/Amazon/Pay/API/Client.php#L421 is empty, then an error is thrown on line 424 for `$contents = file_get_contents($key_spec);`: ``` main.CRITICAL: Warning: file_get_contents(): Filename cannot be empty in vendor/amzn/amazon-pay-api-sdk-php/Amazon/Pay/API/Client.php on line 424 ```