"etc." is a big term.
I'd be leery of searching the Mac for every possible empty folder - there can be empty folders in app bundles, data libraries, the OS, etc. that may still be necessary for proper function.
It's not too hard to search for empty folders in specific locations, like Desktop and Documents - that's where you'd have been likely to create folders that then went unused. Go into one of those folders, set to List View, click on the "gear" icon at the top of the Finder window > Select Show View Options > Select Calculate all sizes. This will show the size of every folder, then sort the contents by size. The empties will float to the top. If you have nested folders, you'll need to lather, rinse, and repeat.