Update NiaAppStateTest to use new InterestsNavKey

The `NiaAppStateTest` is updated to align with the changes in `InterestsNavKey`, which now requires a `topicId` argument. The test now uses `InterestsNavKey(null)` to correctly represent the top-level interests destination.

app/src/testDemo/kotlin/com/google/samples/apps/nowinandroid/ui/NiaAppStateTest.kt

@@ -117,7 +117,7 @@ class NiaAppStateTest {
 
         assertEquals(3, navigationState.topLevelKeys.size)
         assertEquals(
-            setOf(ForYouNavKey, BookmarksNavKey, InterestsNavKey),
+            setOf(ForYouNavKey, BookmarksNavKey, InterestsNavKey(null)),
             navigationState.topLevelKeys,
         )
     }