Fix fill_value=null zarr v2 to v3 conversion for string dtypes

[observingClouds]

Adds support to convert unset/null fill values from zarr format 2 metadata to format 3 for string types.

• Closes Can't concat virtual datasets without dropping CF variables  #826 (at least the dtype part)
• Tests added
• Tests passing
• Full type hint coverage
• Changes are documented in docs/releases.rst
  - [ ] New functions/methods are listed in api.rst
  - [ ] New functionality has documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.11%. Comparing base (c67dcc1) to head (bc8c977).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #845      +/-   ##
==========================================
- Coverage   89.22%   89.11%   -0.12%     
==========================================
  Files          34       34              
  Lines        1987     1985       -2     
==========================================
- Hits         1773     1769       -4     
- Misses        214      216       +2     

Files with missing lines	Coverage Δ
virtualizarr/parsers/zarr.py	97.46% <100.00%> (-1.29%)	⬇️

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[maxrjones]

thanks for working on this, please feel welcome to ping whenever you need a review

[observingClouds]

@maxrjones and others please have a look now. I am not sure if I did the casting correctly to fix the mypy error, but all is passing now. I have also tested that my test is failing with the previous version.