Move away from relying on JS truthiness

See PR #3535 for more context.

I think we should be explicit in our conditional statements instead of relying on truthiness. This should limit gotchas such as the one referenced above.

let a: ReadonlyArray<number> | null = null
let s: string | undefined = undefined

if (a) { }
if (a != null) { } //preferred

a = [0, 1, 2, 3]

if (a.length) { }
if (a.length > 0) { } //preferred

[j-f1]

Aha! https://palantir.github.io/tslint/rules/strict-boolean-expressions/

[iAmWillShepherd]

Closing this out since the team has started being explicit with null and undefined checks.