ut: add null test #6002

Job	Run time
Build source distribution	19s
determine-arm64-runner	1s
Build wheels for cp311-manylinux_x86_64	5m 37s
Build wheels for cp311-manylinux_x86_64	5m 44s
Build wheels for cp311-macosx_x86_64	7m 20s
Build wheels for cp311-macosx_arm64	3m 16s
Build wheels for cp311-win_amd64	4m 46s
Build wheels for cp310-manylinux_aarch64	5m 33s
build_pypi_index	11s
build_docker (12)	4m 14s
build_docker (_cu11, 11)	3m 49s
upload_pypi	0s
deploy_pypi_index	0s
Pass testing build wheels	1s
	40m 51s

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