As far as I know Opus does not have a method to find empty subdirectories by searching for size as I think every directory has a minimum size assigned to it by Windows.
However if you're on speaking terms with batch files, you could create one with the following script and launch it from an Opus button to return a list of empty subdirectories.
@rem BEGIN BATCH FILE
for /f "delims=*" %%a in ('dir /a:d/b/s') do (
dir "%%a" /a:-d >nul 2>nul
if errorlevel 1 echo %%a
)
pause
rem END BATCH FILE