Fix false-negative for CanBeNonNullable

[VitalyVPinchuk]

For #4492
Report code smell when null-check expression returns 'Unit'

[marschwar]

How about this example? If there is any side effect before the guard statement it should not report an issue, right?

fun foo(a: Int?) {
    println("side effect")
    if (a == null) return
    println(a)
}

[VitalyVPinchuk]

Hmm... I didn't think of it.
We'll lose part of the side effect if we make parameter non-nullable. Probably, it's better not to report such a case...
Give your thoughts, please

[marschwar]

That was exactly my thought. I would not report if there is any side effect before the guard statement. Since detecting if it really is a side effect would be hard, I would not allow anything before the guard.

[VitalyVPinchuk]

You're right. I'll fix it.