Fix shape inference for X=None

[flying-sheep]

• Fixes parts of Allowing .X to be None #467
• Tests added
• Release note added (or unnecessary)

TODO:

• Views (basically just check that .X = None doesn't change other view behavior, a[idx].X doesn't do anything weird, and copying views still works)
  unclear if that’s covered @ivirshup can you clarify the scope of this?
• Figure out inference of AnnData shape when X is None

Later:

• Backed mode
• Concatenation
• IO tests
• Specialized writers like to_csvs
• Raw

[codecov]

Codecov Report

Merging #1121 (e1c19c1) into main (2c8759d) will decrease coverage by 2.07%.
The diff coverage is 97.14%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1121      +/-   ##
==========================================
- Coverage   84.75%   82.68%   -2.07%     
==========================================
  Files          36       36              
  Lines        5149     5153       +4     
==========================================
- Hits         4364     4261     -103     
- Misses        785      892     +107     

Flag	Coverage Δ
gpu-tests	?

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Files Changed	Coverage Δ
anndata/_core/anndata.py	83.60% <96.96%> (+0.80%)	⬆️
anndata/_core/raw.py	79.28% <100.00%> (-4.17%)	⬇️

... and 6 files with indirect coverage changes

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[ivirshup]

Mostly minor changes, otherwise good.

For the subsetting + views, I think I was concerned about aligned mappings + raw with various kinds of subsetting.