Using ArrayV3Metadata, kerchunk fill_value will always be coerced to a value, even though `null` is a valid fill_value for Zarr V2

[abarciauskas-bgse]

This issue follows when #453 is merged
🚧 If the fill_value passed to ArrayV3Metadata is None, ArrayV3Metadata will automatically coerce the fill_value to the default of the data type. For example, fill_value will be 0 if None is passed. This means that writing back to v2 metadata will generate a fill_value of the default for the data type, rather than set it as null, which is valid for Zarr V2. See also pydata/xarray#5475 (comment)

Originally posted by @abarciauskas-bgse in #429 (comment)