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

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
```

Start in Amazon/Pay/API/Client.php around lines 421-424 and trace how the private_key configuration is supplied before file_get_contents is called. Reproduce the empty-key warning and determine the intended behavior for a missing private key; done means the configuration no longer produces this warning and the resulting behavior is covered by the project's existing checks.

Written by the indexing model from the issue text.