fix chainNullableK from option

[bwlt]

add missing NonNullable

Why:

type A = {
  k?: Array<'hello' | 'world'> | undefined
}

function program<K extends keyof A>(
  k: K,
  fa: O.Option<A>
): NonNullable<A[K]>[number] | undefined {
  // Type 'unknown' is not assignable to type 'NonNullable<A[K]>[number] | undefined'.
  //  Type 'unknown' is not assignable to type 'NonNullable<A[K]>[number]'.
  //    Type 'unknown' is not assignable to type 'string'.ts(2322)
  // |
  return pipe(
    fa,
    O.chainNullableK((a) => a[k]),
    O.chain(A.head),
    O.toUndefined
  )
}

This code does not compile. The expected result should be

function program<"k">(k: "k", fa: O.Option<A>): "hello" | "world" | undefined