fix(useScroll): types for ref include null and undefined

[josephfh]

The problem:

With Vue composition API the ref is null or undefined intially. There is a type error as these are not accepted

Example

const someEl = ref<HTMLElement | null>(null)
const { y } = useScroll(someEl)

Error:
Argument of type 'Ref<HTMLElement | null>' is not assignable to parameter of type 'MaybeRef<Window | HTMLElement | Document | SVGElement>'.

Fix:

Add null and undefined as accepted types, just like with useScroll() and other vueuse functions