Steps to reproduce:
1) Open Nautilus.
2) Make sure View -> Side Pane is ticked.
3) In the Side Pane, make sure the "Tree" view is selected.
4) Click the drill-down / expander triangle next to a directory that contains subdirectories.
5) This expands to show the subdirectories. Nice.
6) Then click the drill-down / expander triangle next to a directory that does not contain any subdirectories.
7) The drill-down / expander triangle goes from a left-facing triangle into a downwards facing triangle, and underneath the directory it says "(Empty)". Not nice.

What would ideally happen instead:
In Step 6), there would be no drill-down / expander triangle next to the directory without subdirectories, since it has no subdirectories, and therefore it is pointless to expand it.

I.e. do not offer the user an action which does nothing. Instead behave more like Windows Explorer, whose tree view gives the user the ability to tell which directories have subdirectories (those with a "+" before their name), and those which do not (those which do not have a "+" before their name).