ValueError: The `GenerateContentResponse::parts()` quick accessor only works for a single candidate,but none were returned

When Gemini blocks a prompt, it returns zero candidates, and calling $response->parts() throws a ValueError.
I think the fix is to guard against empty candidates in GeminiProvider.php.

Error:
ValueError: The `GenerateContentResponse::parts()` quick accessor only works for a single candidate,but
none were returned. Check the `GenerateContentResponse::$promptFeedback` to see if the prompt was blocked. in
Gemini\Responses\GenerativeModel\GenerateContentResponse->parts() (regel 47 van
/dev/vendor/google-gemini-php/client/src/Responses/GenerativeModel/GenerateContentResponse.php).