Fix prompt incorrectly set to empty when suffix is empty string

[Yiximail]

While using the qwen model, I use the template from its tokenizer_config.json.

"{% for message in messages %}{% if loop.first and messages[0]['role'] != 'system' %}{{ '<|im_start|>system\nYou are a helpful assistant<|im_end|>\n' }}{% endif %}{{'<|im_start|>' + message['role'] + '\n' + message['content']}}{% if (loop.last and add_generation_prompt) or not loop.last %}{{ '<|im_end|>' + '\n'}}{% endif %}{% endfor %}{% if add_generation_prompt and messages[-1]['role'] != 'assistant' %}{{ '<|im_start|>assistant\n' }}{% endif %}"

In this case, the suffix is an empty string, which len is 0.

The execution of prompt[:-0], will incorrectly set the prompt to an empty string.

The conversation works fine, but use "continue" will get an error.

Checklist:

• I have read the Contributing guidelines.