fix wasteful to_python() calls checking for undefined

[davidhewitt]

Change Summary

Calling .to_object() on JSON inputs is potentially expensive and they will NEVER be PydanticUndefined, so this is potentially a massive performance win on JSON validation...

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

[codspeed-hq]

CodSpeed Performance Report

Merging #1580 will improve performances by 49.85%

_{Comparing dh/perf-howler (d219006) with main (730adc3)}

Summary

⚡ 1 improvements
✅ 156 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dh/perf-howler	Change
⚡	test_complete_core_root_json	667.6 µs	445.5 µs	+49.85%

[sydney-runkle]

Looks like we got some little 1-2% perf boosts!

[davidhewitt]

Haha check again 😂