id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
394423623,MDU6SXNzdWUzOTQ0MjM2MjM=,2634,Opening a file with open_mfdataset fails if dask is not installed: ModuleNotFoundError: No module named 'dask.base',226037,closed,0,,,3,2018-12-27T15:33:27Z,2023-12-05T21:43:15Z,2023-12-05T21:43:15Z,MEMBER,,,,"#### Problem description

Opening a file with `open_mfdataset` fails if *dask* is not installed:
```
>>> xr.open_mfdataset('tests/sample-data/era5-levels-members.nc')
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
~/devel/MPY/xarray/xarray/backends/api.py in open_mfdataset(paths, chunks, concat_dim, compat, preprocess, engine, lock, data_vars, coords, autoclose, parallel, **kwargs)
    626 
--> 627     datasets = [open_(p, **open_kwargs) for p in paths]
    628     file_objs = [getattr_(ds, '_file_obj') for ds in datasets]
    629     if preprocess is not None:

~/devel/MPY/xarray/xarray/backends/api.py in open_dataset(filename_or_obj, group, decode_cf, mask_and_scale, decode_times, autoclose, concat_characters, decode_coords, engine, chunks, lock, cache, drop_variables, backend_kwargs)
    340 
    341         with close_on_error(store):
--> 342             return maybe_decode_store(store)
    343     else:
    344         if engine is not None and engine != 'scipy':

~/devel/MPY/xarray/xarray/backends/api.py in maybe_decode_store(store, lock)
    276 
    277         if chunks is not None:
--> 278             from dask.base import tokenize
    279             # if passed an actual file path, augment the token with
    280             # the file modification time

ModuleNotFoundError: No module named 'dask.base'
```

#### Reason

The reason is that the test in `maybe_decode_store` is for `None` but `open_mfdataset` in line 614 sets `chunks` to `{}` if it evaluates to `False`.

In fact setting line https://github.com/pydata/xarray/blob/master/xarray/backends/api.py#L614 as follows solves the problem:
```
    open_kwargs = dict(engine=engine, chunks=chunks, lock=lock,
                       autoclose=autoclose, **kwargs)
```
I could not run the tests to see what breaks with this change as running `pytest` without *dask* fails with:
```
../cpython3/lib/python3.7/site-packages/py/_path/local.py:668: in pyimport
    __import__(modname)
xarray/tests/__init__.py:105: in <module>
    if LooseVersion(dask.__version__) < '0.18':
E   AttributeError: module 'dask' has no attribute '__version__'
```","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/2634/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,not_planned,13221727,issue