An empty state inherits the container's duties

A header composable is rarely just a header. It usually also carries the controls that change what the screen is showing — a period navigator, a filter chip row, a sort control. Swapping the whole thing for "nothing here" therefore removes the only way to reach a state that does have something, and the user is parked.

when {
    topItems is Resource.Loading -> LoadingHeader(headerHeight)   // FIRST, always
    topItem == null              -> EmptyPeriodHeader(uiState, isPortrait, onStep)
    isPortrait                   -> PortraitHeader(topItem, headerHeight, …)
    else                         -> LandscapeHeader(uiState, topItem, onStep, …)
}