Better generic argument for default Thunk Middleware

[phryneas]

This should fix #321

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[markerikson]

As usual, I don't know enough to say if this is actually an improvement or not, so I'll go with your expertise here :)

Two additional things that would potentially need to be updated accordingly:

• The Redux core "Usage with TS" page
• The RTK "Advanced Tutorial" example repo. This one's more complicated, unfortunately, because the repo and the sandboxes based on a specific commit history. We'd really need to go rewrite the commit history with this one change. I've done that a few times already, but it's a pain.

[phryneas]

Okay, should now work with null, undefined and unknown in the AppThunk parameter. As for documentation updates, I guess I have covered everything in various PRs.

See #348 and reduxjs/redux#3693