add `nullishAsync` API reference #1720

Job	Run time
Install packages	18s
Run ESLint in library	45s
Run Vitest in library	33s
Build and publish library	40s
Run Prettier in library	35s
Run ESLint in packages/to-json-schema	26s
Run Vitest in packages/to-json-schema	20s
Run Prettier in packages/to-json-schema	19s
Run ESLint in website	26s
Run Prettier in website	44s
	5m 6s

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