Allow optional DataGrid props to be null

[nstepien]

Allowing null for optional props means this pattern won't be necessary anymore:

<DataGrid optionalProp={maybeNull ?? undefined} />

??, ??=, ?., and != null makes this trivial thankfully.
Only downside I found was with default function params, param = default doesn't replace nulls.
Non-ts users should get fewer surprises as well.

I've also fixed some missed spots with the new K generic.

Column props, and maybe other props will be done in separate PRs.

[nstepien]

There is a little bit of nuance here vs default params, if a param isn't defaulted in the parameter list, then it keeps its original type in functions, as TS cannot know that those functions are only called after the ??= line. I wish we could use ??= in the param list. It also means they can be re-assigned to null or undefined if we're not careful.

function fnA(param = 1) {
  param; // type; number

  function test() {
    param; // type: number
  }
}

function fnB(param: number | undefined | null) {
  param; // type: number | undefined | null
  param ??= 1;
  param; // type: number

  function test() {
    param; // type: number | undefined | null
    // this is because we could call `test` before the line where we set it to 1, or it can be reassigned to `null | undefined`
  }
}