Zarr: missing fill_value on creation

[rouault]

netcdf-c master at 84f0696 , on Ubuntu 20.04

given groups.h5,
./ncdump/nccopy groups.h5 "file://groups_regular.zarr#mode=zarr,file"

generates groups_regular.zarr/MyGroup/dset1/.zarray with

{"zarr_format": 2,"shape": [3,3],"dtype": ">i4","chunks": [3,3],"order": "C","compressor": null,"filters": null}

There's no fill_value key, which is required by the Zarr specification (https://zarr.readthedocs.io/en/stable/spec/v2.html#metadata), and should be at null

This causes the Zarr python implementation to reject it (as well as the new GDAL Zarr driver: https://gdal.org/drivers/raster/zarr.html):

$ python
Python 3.8.10 (default, Jun  2 2021, 10:49:15) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import zarr
>>> z = zarr.open('/home/even/netcdf-c/build/groups.zarr')
>>> z['/MyGroup/dset1']
Traceback (most recent call last):
  File "/home/even/.local/lib/python3.8/site-packages/zarr/meta.py", line 43, in decode_array_metadata
    fill_value = decode_fill_value(meta['fill_value'], dtype)
KeyError: 'fill_value'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/even/.local/lib/python3.8/site-packages/zarr/hierarchy.py", line 341, in __getitem__
    return Array(self._store, read_only=self._read_only, path=path,
  File "/home/even/.local/lib/python3.8/site-packages/zarr/core.py", line 159, in __init__
    self._load_metadata()
  File "/home/even/.local/lib/python3.8/site-packages/zarr/core.py", line 176, in _load_metadata
    self._load_metadata_nosync()
  File "/home/even/.local/lib/python3.8/site-packages/zarr/core.py", line 191, in _load_metadata_nosync
    meta = decode_array_metadata(meta_bytes)
  File "/home/even/.local/lib/python3.8/site-packages/zarr/meta.py", line 57, in decode_array_metadata
    raise MetadataError('error decoding metadata: %s' % e)
zarr.errors.MetadataError: error decoding metadata: 'fill_value'
>>>