Fix get_prompt_and_rejected in CohereMiracljaqueries2212Dataset

[kajyuuen]

negative_passages length for CohereMiracljaqueries2212Dataset may be 0.
Below is an example.
https://huggingface.co/datasets/Cohere/miracl-ja-queries-22-12/viewer/Cohere--miracl-ja-queries-22-12/train?row=2

[conglongli]

Hi @kajyuuen thank you for the contribution. The formatting test is failing, please see my comment at #597 (comment) about how to fix it. Thank you.

Also regarding these Japanese datasets, when I found these datasets I did some basic step-1 training tests using them (https://github.com/microsoft/DeepSpeedExamples/blob/master/applications/DeepSpeed-Chat/training/step1_supervised_finetuning/training_scripts/other_language/run_japanese.sh). However, the generated answers were not that great, potentially due to the quality of model/data (or both). So please take that as a caveat. We welcome your contributions if you find any better model/data/training recipe.

[kajyuuen]

@conglongli
Thank you for your comment and advice! I have reformatted it.