assertion error on digest type

while trying to figure out
- #1549
on drogon, got in

```
2024-12-12T09:24:00-0500 [ERROR ] dandi 3434539:140629314469952 Caught while downloading sub-I48/ses-SPIM/micr/sub-I48_ses-SPIM_sample-BrocaAreaS03_stain-Somatostatin_SPIM.ome.zarr: Traceback (most recent call last):
File "/home/yoh/proj/dandi/dandi-cli/dandi/download.py", line 397, in _download_generator_guard yield from generator
File "/home/yoh/proj/dandi/dandi-cli/dandi/download.py", line 1043, in _download_zarr zarr_checksum = asset.get_digest().value ^^^^^^^^^^^^^^^^^^ File "/home/yoh/proj/dandi/dandi-cli/dandi/dandiapi.py", line 1513, in get_digest return Digest(algorithm=algorithm, value=self.get_raw_digest(algorithm)) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "/home/yoh/proj/dandi/dandi-cli/dandi/dandiapi.py", line 1499, in get_raw_digest assert isinstance(digest, str)
AssertionError
```

and of cause we do not know what type it is ...