fix(union_serializer): do not raise warnings in nested unions #5327

Job	Run time
build sdist	19s
build on ${{ matrix.os }} (${{ matrix.target }} - ${{ matrix.interpreter \|\| 'all' }}${{ (((matrix.os == 'linux') && format(' - {0}', (((matrix.manylinux == 'auto') && 'manylinux') \|\| matrix.manylinux))) \|\| '') }})	0s
build pgo-optimized on ${{ matrix.os }} / ${{ matrix.interpreter }}	0s
rust benchmarks	3m 7s
build-wasm-emscripten	2m 38s
coverage	3m 4s
lint	1m 7s
test-debug 3.13	54s
test MSRV	2m 51s
test-debug pypy3.10	1m 35s
test on ubuntu	2m 23s
test-pydantic-integration	1m 58s
test on macos	1m 23s
test 3.8	1m 20s
test on windows	4m 33s
test 3.9	1m 34s
test 3.10	2m 15s
test 3.11	2m 3s
test 3.12	2m 14s
test 3.13	1m 49s
test pypy3.9	1m 41s
test pypy3.10	2m 33s
test build on ${{ matrix.target }}-${{ matrix.distro }}	0s
test build on ${{ matrix.os }}	0s
inspect-pypi-assets	0s
check	2s
release	0s
	41m 23s

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