Fixing: Error handling for non-English characters

[tknrych]

Hello,

Thank you very much for providing such a useful tool. To address the issue where the process stops when the base sentence for augmentation contains non-English characters like Japanese texts and so on, I have added one modification to the processing. I would greatly appreciate it if you could review the following pull request.

Thank you for your time.

If there are no available words in the sentence, the process exits this function.