If you're using 8.5, you could reverse this, and just use the highlight expert.
Set the default background colour to whatever you want the null background colour to be.
Set the highlight expert to change the background colour to white where the value = 0, or is < or > 0.
When it's null, the default colour of, say, red will show through. Otherwise, the background is white.
You have to do this bass-ackwards like this, because you can't manipulate nulls in the highlight expert.
Naith