Remember narrowed types in conditions

[ondrejmirtes]

Given this template:

<!-- ko if: alertErrorMessage() !== null -->
    <!-- ko text: alertErrorMessage --><!-- /ko -->
<!-- /ko -->

KOLint still complains with an error message like this:

Argument of type 'string | null' is not assignable to parameter of type 'string'.

The code should be compiled in such a way to make TypeScript understand that alertErrorMessage cannot be null inside the if.