Even in the overloads, state that it can accept `null` and
`undefined`, in order to ensure easy composition with `async`.
Additionally, change the implementation to return `null` on an
`undefined` input, for consistency with other pipes.
BREAKING CHANGE:
The `slice` pipe now returns `null` for the `undefined` input value,
which is consistent with the behavior of most pipes. If you rely on
`undefined` being the result in that case, you now need to check for it
explicitly.
PR Close#37447
4744c229db