[BUG]unexpected behavior on qwen 2.5 series templates

[xypan0]

Describe the bug
When using qwen 2.5 series templates, tokenizer.apply_chat_template throw Jinja error

To Reproduce
Steps to reproduce the behavior:

1. Create a conversation dataset without a system prompt
2. use qwen 2.5 series template
3. The error occurs.

Additional context
https://github.com/OptimalScale/LMFlow/blob/main/src/lmflow/tokenization/hf_decoder_model.py#L160
Here the code does not handle None system prompt properly.

[wheresmyhair]

Thanks for reporting this issue ❤ , and should be fixed in PR #935 .