fix BenchmarkDatasetConfig sub_dataset nullable

[qjiang002]

For datasets without sub_dataset, the sub_dataset property should be nullable, otherwise it will raise error when retrieving datasets for benchmark page.

[pfliu-nlp]

LGTM

[OscarWang114]

Thanks for fixing this! I think there are a couple of fields that should be marked as nullable as well, but we can save those for a future PR. I’ll also work on it.

[qjiang002]

Thanks for fixing this! I think there are a couple of fields that should be marked as nullable as well, but we can save those for a future PR. I’ll also work on it.

I record this in issue #525. Thanks for help.