simplify instantiation of undefined type

[samuelcolvin]

and store undefined on the relevant validators.

[codecov]

Codecov Report

Merging #1157 (685b207) into main (8030c5a) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1157      +/-   ##
==========================================
- Coverage   89.94%   89.94%   -0.01%     
==========================================
  Files         106      106              
  Lines       16438    16437       -1     
  Branches       36       36              
==========================================
- Hits        14785    14784       -1     
  Misses       1646     1646              
  Partials        7        7              

Files	Coverage Δ
src/argument_markers.rs	86.44% <ø> (-0.66%)	⬇️
src/validators/model.rs	97.84% <100.00%> (+<0.01%)	⬆️
src/validators/with_default.rs	99.09% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #1157 will not alter performance

_{Comparing simplify-undefined (685b207) with main (8030c5a)}

Summary

✅ 142 untouched benchmarks