Fix notification stack height returning shelfHeight when there's no notficiations at all

Though we can get shelfHeight from NotifStackCalculator, but it should
not happen according to the comment, so we check if there's
notifications by activeNotficationsInteractor

Bug: 404689882
Flag: com.android.systemui.shared.extended_wallpaper_effects
Test: atest SharedNotificationContainerViewModelTest

Change-Id: Ida087275758cba4b0d507a50f49d91fe262046d2