setSize on useSWRInfinite when page index is null output object as data instead of undefined

[mhsattarian]

Bug report

Description / Observed Behavior

When page index (first parameter of useSWRInfinite) is null and setSize method is called, data instead of being undefined (as expected) becomes an object as below:

{
	data: undefined,
    "force": false
}

Expected Behavior

It should output undefined as data.

Repro Steps / Code Example

use the swr-infinite CodeSandbox from the docs and change version to 0.5.6.
or use this CodeSandbox.

Additional Context

this was ok in version 0.3.0 as I tested in swr-infinite CodeSandbox, but it's not now in 0.5.6.

I hadn't investigated the problem, but I think there might be an issue with this line:

swr/infinite/index.ts

Line 110 in c677d04

let pageData = cache.get(pageKey)

I guess it should be like this?:

const { data: pageData, force } = cache.get(pageKey)

[shuding]

This bug should have been fixed in the latest beta, I cannot reproduce it here: https://codesandbox.io/s/swr-infinite-forked-mx7wc.