Fix optional download_size
Should fix https://huggingface.co/datasets/Open-Orca/OpenOrca dataset viewer.

The dataset was stream converted completely so partial is False but the download_size is still None because streaming was used.
Fix optional download_size: Should fix https://huggingface.co/datasets/Open-Orca/OpenOrca dataset viewer.

The dataset was stream converted completely so partial is False but the download_size is still None because streaming was used.