Fix empty examples in xtreme dataset for bucc18 config
_The documentation is not available anymore as the PR was closed or merged._ I guess the report link is this instead: https://huggingface.co/datasets/xtreme/discussions/1
As reported in https://huggingface.co/muibk, there are empty examples in xtreme/bucc18.de

I applied your fix @mustaszewski

I also used a dict to make the dataset generation much faster