NonMatchingChecksumError when checksum is None
I ran into the following error when adding a new dataset:

```bash
expected_checksums = {'https://adversarialglue.github.io/dataset/dev.zip': {'checksum': None, 'num_bytes': 40662}}
recorded_checksums = {'https://adversarialglue.github.io/dataset/dev.zip': {'checksum': 'efb4cbd3aa4a87bfaffc310ae951981cc0a36c6c71c6425dd74e5b55f2f325c9', 'num_bytes': 40662}}
verification_name = 'dataset source files'

    def verify_checksums(expected_checksums: Optional[dict], recorded_checksums: dict, verification_name=None):
        if expected_checksums is None:
            logger.info("Unable to verify checksums.")
            return
        if len(set(expected_checksums) - set(recorded_checksums)) > 0:
            raise ExpectedMoreDownloadedFiles(str(set(expected_checksums) - set(recorded_checksums)))
        if len(set(recorded_checksums) - set(expected_checksums)) > 0:
            raise UnexpectedDownloadedFile(str(set(recorded_checksums) - set(expected_checksums)))
        bad_urls = [url for url in expected_checksums if expected_checksums[url] != recorded_checksums[url]]
        for_verification_name = " for " + verification_name if verification_name is not None else ""
        if len(bad_urls) > 0:
            error_msg = "Checksums didn't match" + for_verification_name + ":\n"
>           raise NonMatchingChecksumError(error_msg + str(bad_urls))
E           datasets.utils.info_utils.NonMatchingChecksumError: Checksums didn't match for dataset source files:
E           ['https://adversarialglue.github.io/dataset/dev.zip']

src/datasets/utils/info_utils.py:40: NonMatchingChecksumError
```

## Expected results
The dataset downloads correctly, and there is no error.

## Actual results
Datasets library is looking for a checksum of None, and it gets a non-None checksum, and throws an error. This is clearly a bug.