There are a few ways (as always with Tableau) you could try.
1. Use custom shapes, and use a blank image, and set the NULL to that image. Let me know if you've not come across custom shapes and I can direct you to some resources
2. Throw the Up/Down/Null dimension onto the colour shelf too...and set the NULL colour to the background colour (eg. white)
Hopefully one of them will do the trick, but if not please post back (with why they aren't working) and I'll have another think!