Serialize Nones

[flying-sheep]

• Fixes .write does not save None values #673
• Added tests
• Added release notes

Rendered release notes

[codecov]

Codecov Report

Merging #999 (c9fede4) into main (8ffe21e) will decrease coverage by 2.13%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #999      +/-   ##
==========================================
- Coverage   85.03%   82.90%   -2.13%     
==========================================
  Files          36       36              
  Lines        5385     5394       +9     
==========================================
- Hits         4579     4472     -107     
- Misses        806      922     +116     

Flag	Coverage Δ
gpu-tests	?

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Files	Coverage Δ
anndata/_io/specs/methods.py	88.05% <100.00%> (+0.07%)	⬆️
anndata/_io/specs/registry.py	95.62% <ø> (-0.06%)	⬇️

... and 7 files with indirect coverage changes

[ivirshup]

As discussed, I don't want to change the on-disk format in the next release. So I'd hold off on this until at least 0.11.

[flying-sheep]

Since 0.10 is released, we can document and then merge this, right?

[ivirshup]

I would like to have proposals for all disk format changes written up and discussed with other implementers before merging them to main.

[flying-sheep]

Sure, we can use this PR as reference implementation.

Which other implementers are there? Is there a channel with the relevant people on some communication medium?