Allow chunks=None for dask-less lazy loading

[LunarLanding]

Otherwise, the preprocess step always gets a single chunk for each variable instead of the xarray lazy accessors; dask can't change the order of slicing / computing in most cases. The default is changed to {} to maintain the default behavior.

• Closes #xxxx
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[keewis]

this might be a duplicate of #5704. If I'm reading that correctly, the "only" thing missing is to add support to concatenation to our lazy indexing machinery.

[LunarLanding]

@keewis thanks, it is a duplicate. it must work in my specific case because I was testing with just one file.