There's a lot of info in these replies, but here's what I did to get rid of my empty import folders, step by step.
1. I deleted the empty folders.
2. I went into Aperture preferences and turned off Automate Import and Automatic Upload.
3. I closed the preferences and quit Aperture.
4. I reopened Aperture.
5. I turned the Automatic Import and Upload features back on (I do like that).
6. I closed the preferences and quit Aperture again.
7. When I reopened Aperture, the empty folders were in the Trash. I emptied the trash and all is right with the world.
So... it seems that emptying and deleting a month of photo stream before the end of the month creates the snafu. I'll try to keep them in place until the end of the month, then bring them into my other folders.
Hope this helps.