fix(python): Make schema picklable

[moritzwilksch]

fixes #17523

during unpickling, an instance of Schema is instantiated without passing the schema argument. In this case, we should create an empty Schema instead of erroring.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.49%. Comparing base (a5724d6) to head (5d2bb5c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17524      +/-   ##
==========================================
+ Coverage   80.47%   80.49%   +0.01%     
==========================================
  Files        1482     1483       +1     
  Lines      195119   195119              
  Branches     2778     2778              
==========================================
+ Hits       157026   157052      +26     
+ Misses      37582    37556      -26     
  Partials      511      511              

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