Improve inefficient retrieval of zarr chunks from the API

We are receiving many requests to the API, from the CLI, of the following format:
```
GET /api/zarr//files/?prefix=0/0/0/13/14/97&download=true
```

Often times, the path provided (`0/0/0/13/14/97`) is itself the only file returned, and as such, could just be retrieved by querying for the "level" above (`0/0/0/13/14`). Really though, every response from that endpoint is itself an object, so there should never be a case where the CLI is trying to determine if a path returned is a directory or a file.

~I believe the code generating these requests lives [here](https://github.com/dandi/dandi-cli/blob/953923aff871fc4dd16e3435427576e48eadb7c4/dandi/dandiapi.py#L1786-L1795)~ See the comment below.

It's possible the requests we're receiving are from a modified version of the CLI, in which case this issue can be closed (if we can truly determine that to be the case).