* fix getSnapshot warning when a selector returns NaN
useSyncExternalStoreWithSelector delegate a selector as
getSnapshot of useSyncExternalStore.
* Fiber's use sync external store has a same issue
* Small nits
We use Object.is to check whether the snapshot value has been updated,
so we should also use it to check whether the value is cached.
Co-authored-by: Andrew Clark <git@andrewclark.io>