Return unmodified schema from MD.schema

[benjeffery]

Fixes #3129

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.94%. Comparing base (fc55565) to head (e043227).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3130   +/-   ##
=======================================
  Coverage   89.94%   89.94%           
=======================================
  Files          29       29           
  Lines       32650    32651    +1     
  Branches     5854     5854           
=======================================
+ Hits        29367    29368    +1     
  Misses       1869     1869           
  Partials     1414     1414           

Flag	Coverage Δ
c-tests	86.66% <ø> (ø)
lwt-tests	80.78% <ø> (ø)
python-c-tests	89.23% <ø> (ø)
python-tests	98.83% <100.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
python/tskit/_version.py	100.00% <100.00%> (ø)
python/tskit/metadata.py	99.00% <100.00%> (+<0.01%)	⬆️

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