fix: handle mismatched dim lengths during rechunking (take 2)

[coroa]

Fixes #395 and #381 .

@lkstrp
We stumbled over your changes in #395 today starting from some custom code.

If ds has an unchunked time dimension you get

chunks = "auto"

and then

chunks[0]

retrieves the letter "a", which breaks n.chunk(...).

I think you meant to check for whether there are chunks (which are always given as a tuple) and then use the leading chunksize in that case.

@geiges Thanks for the report.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.