Transforming a nullable string into a potentially undefined string

pipe(
  S.nullish(S.string),
  S.transform(
    S.optional(S.string), 
    s => s ?? undefined,
    identity
  )
)

pipe(
  S.nullish(S.string),
  S.transform(
    S.union(S.string, S.undefined), 
    s => s ?? undefined,
    identity
  )
)

├─ Union member
│  └─ Expected null, actual "https://example.com"
├─ Union member
│  └─ Expected undefined, actual "https://example.com"
└─ Union member
   └─ ...