How to check for value 0 in @if template flow syntax

I have a problem with @if in the Angular template flow syntax.

A value is available in an RxJs Observable. So the async pipe helps and the value is assigned to a variable.

@if (currentPageNumber$ | async; as currentPageNumber) {
// currentPageNumber is number

For value 0 the if statement is not valid. So I exclude only null values… but now the value of currentPageNumber is boolean.

@if ((currentPageNumber$ | async) !== null; as currentPageNumber) {
// currentPageNumber is boolean

How can I check against null but keep my variable with the value of the stream?