When providing fetcher: null , middleware is defaulting it to undefined

[jquintozamora]

Bug report

We're building a custom middleware were we interact with fetcher.
We check if fetcher === null, but we receive fetcher as undefined inside middleware callback.

 const myCustomMiddleware = (useSwrNext) =>
  (key, fetcher, config) => {
    const extendedFetcher =  fetcher === null ? null : myFetcher
    ....

The issue is that fetcher comes undefined even if we set it as null in our useSWRImmutable("myKey", null, options)
Types doesn't allow us to define fetcher as undefined in useSWRInmutable.

Expected Behavior

Middleware returns fetcher as null when we set it null in useSWRInmutable

SWR version.
"swr": "^1.3.0"

[koba04]

@jquintozamora Thank you! I've created a PR to fix this.