Stop using null unnecessarily in source code

[JoshuaKGoldberg]

There are many places in source code that use null and undefined interchangeably. As a code cleanup task, it'd be nice to consolidate on only using undefined whenever possible. This is what most libraries do, including TSLint and TypeScript.

[reduckted]

There's a number of places in the code where it's using x != null or x !== null. For example:

https://github.com/Microsoft/tslint-microsoft-contrib/blob/7e87ab24758fef21611d13091e74c55c4fc70a4e/src/chaiPreferContainsToIndexOfRule.ts#L64-L66

When dealing with strings, x === undefined would be required to keep the same behavior, but when dealing with objects, which of these is the preferred style?:

if (x) {
}

or

if (x !== undefined)

[JoshuaKGoldberg]

which of these is the preferred style

It'd be nice to eventually also enable strict-boolean-expressions (#491), so the x !== undefined would be nice in that regard. I've been bitten by x equaling 0 enough times to be wary of if (x).

However, if it makes it easier and less prone to bugs _(considering the many places where variables are * | null | undefined, if (x) should be fine to start here. We can always do another pass-through before enabling strict-boolean-expressions.