collection-view: Check subpage is open before consuming input

Currently when an empty collection is opened the is_collection_empty property is set to false. This property alone was used to check if search should be disabled. That means whenever an empty collection was viewed and exited, search would no longer work because is_collection_ empty prop is still true. So we now check whether collection subpage is open before checking if the current collection is empty to consume the input, so that search is disabled in those situations.

This bug can't be seen as of now because we currently only support non-user collections which are hidden when empty.

See Also: https://gitlab.gnome.org/GNOME/gnome-games/-/merge_requests/416#note_887295