Rechunker broken with new xarray and zarr

[juliettelavoie]

It looks like rechunker was broken by new versions of xarray for rechunking zarr written to disk.

Minimal working example:

import xarray as xr
from rechunker import rechunk
ds = xr.tutorial.open_dataset("air_temperature")
ds.to_zarr('ds1.zarr')
ds = xr.open_zarr('ds1.zarr')
array_plan = rechunk(
    ds, 
    target_chunks={"time": 2920, "lat": 25, "lon": 1},
    max_mem='1GB',
    target_store='ds2.zarr',
    temp_store='temp'
)

This worked with version 2024.9.0, but with version 2024.10.0 and up, I get the following error:

Traceback (most recent call last):
  File "/exec/jlavoie/.conda/xscen-dev/lib/python3.12/site-packages/IPython/core/interactiveshell.py", line 3577, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-37-1d0e2ad48247>", line 4, in <module>
    array_plan = rechunk(
                 ^^^^^^^^
  File "/exec/jlavoie/.conda/xscen-dev/lib/python3.12/site-packages/rechunker/api.py", line 303, in rechunk
    copy_spec, intermediate, target = _setup_rechunk(
                                      ^^^^^^^^^^^^^^^
  File "/exec/jlavoie/.conda/xscen-dev/lib/python3.12/site-packages/rechunker/api.py", line 447, in _setup_rechunk
    variable_encoding = extract_zarr_variable_encoding(
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/exec/jlavoie/.conda/xscen-dev/lib/python3.12/site-packages/xarray/backends/zarr.py", line 473, in extract_zarr_variable_encoding
    chunks = _determine_zarr_chunks(
             ^^^^^^^^^^^^^^^^^^^^^^^
  File "/exec/jlavoie/.conda/xscen-dev/lib/python3.12/site-packages/xarray/backends/zarr.py", line 329, in _determine_zarr_chunks
    for zchunk, dchunks, interval, size in zip(
                                           ^^^^
TypeError: 'NoneType' object is not iterable

Note that if I don't write ds1 to disk, there is no error.