fix nullable value handling (#142) #145

Job	Run time
build_asan	4m 47s
build (ubuntu-latest, 3.10.11, false)	34s
build (ubuntu-latest, 3.10.11, true)	22s
build (ubuntu-latest, 3.8.17, false)	19s
build (ubuntu-latest, 3.8.17, true)	27s
build (ubuntu-20.04, 3.6.8, false)	24s
build (ubuntu-20.04, 3.6.8, true)	33s
build (ubuntu-latest, pypy-3.7, false)	27s
build (ubuntu-latest, pypy-3.7, true)	35s
build (macos-latest, 3.10.11, false)	3m 26s
build (macos-latest, 3.8.17, false)	3m 52s
build (macos-latest, 3.6.8, false)	58s
build (macos-latest, pypy-3.7, false)	33s
build (windows-latest, 3.10.11, false)	2m 6s
build (windows-latest, 3.8.10, false)	3m 57s
build (windows-latest, 3.6.8, false)	5m 4s
build (windows-latest, pypy-3.7, false)	1m 55s
	30m 19s

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