Add null check section in type guard page

[golopot]

https://github.com/basarat/typescript-book/blob/master/docs/types/typeGuard.md

Cases like this:

function foo(a?: number, b?:number) {
  if (a) {
    // a is number now.
  }

  while (!b) {
    ...
  }
  // b is number now
}

[basarat]

Thanks. Added a version 🌹

Minor, careful with falsy values e.g.

function foo(a?: number, b?:number) {
  if (a) {
    // a is number now.
  }
  else {
    // a is `number?` (instead of `undefined`) as 0 (a member of number) is also falsy. 
  }
 
}