feat: allow reading nullable arrays without mask on disk

[flying-sheep]

• Closes #
• Tests added
• Release note added (or unnecessary)

Discovered by @katosh in #2272 (comment)

If the mask array is missing, we try to create a MaskedArray without mask. This draft shows how actually supporting that might look like.

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.65%. Comparing base (a892252) to head (81a5435).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/anndata/experimental/backed/_lazy_arrays.py	84.21%	3 Missing ⚠️
src/anndata/_io/specs/methods.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2279      +/-   ##
==========================================
- Coverage   86.72%   84.65%   -2.08%     
==========================================
  Files          46       46              
  Lines        7196     7207      +11     
==========================================
- Hits         6241     6101     -140     
- Misses        955     1106     +151     

Files with missing lines	Coverage Δ
src/anndata/_io/utils.py	78.01% <100.00%> (+0.47%)	⬆️
src/anndata/_io/specs/methods.py	90.50% <92.85%> (-0.43%)	⬇️
src/anndata/experimental/backed/_lazy_arrays.py	91.05% <84.21%> (-0.68%)	⬇️

... and 6 files with indirect coverage changes