Fix bug in v2 zarr parser which interprets empty dimension list as truthy

[jbusecke]

What I did

See #935

Acceptance criteria:

• Closes Zarr Parser fails to parse v2 store with scalar variable in latest main #935
• Tests added
• Tests passing
• No test coverage regression
• Full type hint coverage
• Changes are documented in docs/releases.md

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.23%. Comparing base (bdc1a3a) to head (512c8d5).
⚠️ Report is 51 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #936   +/-   ##
=======================================
  Coverage   89.23%   89.23%           
=======================================
  Files          33       33           
  Lines        2025     2025           
=======================================
  Hits         1807     1807           
  Misses        218      218           

Files with missing lines	Coverage Δ
virtualizarr/parsers/zarr.py	97.50% <100.00%> (ø)

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