Don't build dummy objects when populating ObTypeLookup

[davidhewitt]

Change Summary

Slight simplification that I noticed while working on #1085.

Related issue number

N/A

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @samuelcolvin

[davidhewitt]

please review

[codecov]

Codecov Report

Merging #1086 (93ae215) into main (be9c21c) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1086      +/-   ##
==========================================
- Coverage   89.62%   89.62%   -0.01%     
==========================================
  Files         106      106              
  Lines       16366    16363       -3     
  Branches       35       35              
==========================================
- Hits        14668    14665       -3     
  Misses       1691     1691              
  Partials        7        7              

Files	Coverage Δ
src/serializers/ob_type.rs	81.02% <100.00%> (-0.23%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #1086 will improve performances by 33.77%

_{Comparing dh/no-dummy-objects (93ae215) with main (be9c21c)}

Summary

⚡ 2 improvements
✅ 138 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dh/no-dummy-objects	Change
⚡	test_core_str	37.9 µs	31.1 µs	+22.06%
⚡	test_validate_literal[json-few_mixed]	37.2 µs	27.8 µs	+33.77%