The ‘Show empty optional fields' function automatically hides empty fields in the ‘Edit Node’ dialog on the ‘Edit Hierarchy’, and ‘View Hierarchy’ pages. However, the empty fields can be made visible, need to click on the checkbox at the bottom of the dialog.
Each time you re-open the ‘Edit Node’ dialog, empty fields will be hidden by default. If you want to see them, you'll need to check the box again.