fix wasteful `to_python()` calls checking for undefined #5521

Job	Run time
build on ${{ matrix.os }} (${{ matrix.target }} - ${{ matrix.interpreter \|\| 'all' }}${{ (((matrix.os == 'linux') && format(' - {0}', (((matrix.manylinux == 'auto') && 'manylinux') \|\| matrix.manylinux))) \|\| '') }})	0s
rust benchmarks	2m 52s
build pgo-optimized on ${{ matrix.os }} / ${{ matrix.interpreter }}	0s
build sdist	22s
build-wasm-emscripten	2m 19s
coverage	1m 6s
lint	54s
test-debug 3.13	47s
test MSRV	3m 22s
test-debug pypy3.10	1m 10s
test on ubuntu	2m 23s
test-pydantic-integration	1m 23s
test on macos	2m 8s
test on windows	4m 36s
test 3.8	1m 37s
test 3.9	1m 49s
test 3.10	1m 51s
test 3.11	1m 39s
test 3.12	1m 43s
test 3.13	2m 3s
test pypy3.9	2m 17s
test pypy3.10	2m 2s
test build on ${{ matrix.os }}	0s
test build on ${{ matrix.target }}-${{ matrix.distro }}	0s
inspect-pypi-assets	0s
check	4s
release	0s
	38m 27s

Provide feedback