feat: add a warning for invalid arguments with suspense mode

[koba04]

Fixes #1359.
I've added a warning for invalid arguments with suspense mode.
The check (!key || !fn) is the same validation used in other places.

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[shuding]

Thanks! 🎉

[shuding]

Gonna merge this for now, we can fix the CI later.

[koba04]

Thank you!

[karol-majewski]

With that change, how are we supposed to fetch conditionally with suspense?

According to the docs, a falsy value should be used when SWR is not supposed to make a call. I did exactly that, and now in v1.0.1 my app doesn't work anymore.

[koba04]

@karol-majewski Thank you for your feedback, I missed the document and that makes sense for me. I think this is a breaking change with the document and I'll create a PR to revert this.
There are some discussions about how suspense mode behaves with nullish values for key and fetcher (#1412), so we should continue discussing about this.

[karol-majewski]

@koba04 Thank you for such a quick reaction! 🙇