[docs] Conditional Fetching - false instead of null?

[Izhaki]

The conditional fetching docs state:

Use null or pass a function as key to conditionally fetch data. If the function throws or returns a falsy value, SWR will not start the request.

And the example goes:

// conditionally fetch
const { data } = useSWR(shouldFetch ? '/api/data' : null, fetcher)

But looking at the code, a false value will do just as much as null; This allows a slightly less verbose syntax:

// conditionally fetch
const { data } = useSWR(shouldFetch && '/api/data', fetcher)

Is null some future-proof value? Should we update the docs?