Allow sortStatus to be undefined

[arielshulman]

First of all thanks for the great work.

I've been using this component and encountered a TS error after I've used sortStatus and onSortStatusChange, while sortStatus could be undefined and it was initialized like this.

Technically it is working as expected right now for me, as sortStatus is set to undefined and the initial view starts with no sorting at all (put aside the TS error).

After understanding the TS error (which was hard with the error plotted by tsc that said something on withBorder 🤷‍♂️), I've thought sortStatus it should considered a feature, not a bug, and just fix the Types that way.

What do you think?

[arielshulman]

Well, this discussion is a bit like #109 (comment), but that solution is also non TS acceptable.