Sidecar JSON file is not recognized by `dandi upload` when the `dataset_description.json` file is missing

## Description
1. When uploading a new or edited JSON file that accompanies a Zarr file in a BIDS inspired dataset, the JSON file does not get recognized by the `dandi upload` command when the `dataset_description.json` file is missing at the top-level directory of the Dandiset. Do you know why this might be the case?
2. The `dataset_description.json` file is required for a BIDS dataset (see [BIDS Specification](https://bids-specification.readthedocs.io/en/stable/modality-agnostic-files.html#dataset-description)), but we don't specify that this is a requirement for a Dandiset.

Thank you.

cc @dstansby @aaronkanzer