Fix adata layers None after anndata X unification

[LucaMarconato]

Failing due to adata allowing finding None among the keys of layers, full discussion here #1119.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.03%. Comparing base (ef757d1) to head (c429089).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1122   +/-   ##
=======================================
  Coverage   92.03%   92.03%           
=======================================
  Files          51       51           
  Lines        7757     7761    +4     
=======================================
+ Hits         7139     7143    +4     
  Misses        618      618           

Files with missing lines	Coverage Δ
src/spatialdata/_core/validation.py	100.00% <100.00%> (ø)

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