Add null to lastJsonMessage type

[s-h-a-d-o-w]

The user has no control over the fact that on first render, lastJsonMessage will be null. And it always is like that (I suspect. Without digging through the code, I don't imagine there's a scenario where the socket is established and sets the value quickly enough for it not to be?).

And so I would argue that the user should have no option of not including null. They might also wrongly assume that undefined is used.

[PrakashC1]

I was surprised — the documentation states the type is T | null, but in the actual library, null isn’t included. However, during the initial load, lastJsonMessage is actually null

After enabling the ESLint rule @typescript-eslint/no-unnecessary-condition, it started flagging all my null checks as unnecessary.

[cadonau]

I was surprised — the documentation states the type is T | null, but in the actual library, null isn’t included. […]

I just noticed (/ran into) the same.