I have Opus 10, but I disabled RAR in Preferences / Zip & Other Archives / Archive and VFS Plugins as I like to use the WinRAR interface.
WinRAR.exe doesn't have a way of creating empty archives. (In fact, WinRAR.exe is downright weird about empty archives: If you delete all the files in an archive it'll delete the archive itself. That ain't right.)
Opus 10 does have a way, but if you want to use WinRAR.exe instead of Opus then you're on your own.