(fix): allow init without X and correct shape inferred

[ilan-gold]

• Closes Initializing AnnData without X but with layers errors #1816
• Tests added
• Release note added (or unnecessary)

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 83.82%. Comparing base (8d7beab) to head (c53c689).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/anndata/_core/anndata.py	90.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1841      +/-   ##
==========================================
- Coverage   86.13%   83.82%   -2.32%     
==========================================
  Files          41       41              
  Lines        6277     6283       +6     
==========================================
- Hits         5407     5267     -140     
- Misses        870     1016     +146     

Files with missing lines	Coverage Δ
src/anndata/_core/anndata.py	82.56% <90.00%> (+0.93%)	⬆️

... and 8 files with indirect coverage changes

[flying-sheep]

There’s a bug with layers. Please fix and add a test inferring from non-square layers to catch that.