I think I figured out a way to do this. If you uncheck 'Suppress Zero-values' , you will first get zeros in your cells with missing data. I needed dashes and not zeros ( but this will also work for spaces).
On the number tab under 'Format Pattern', I originally had #,##0;(#,##0) (in order to get parens around negative values). There is no final semicolon as you can see. To get the dashes, I added a semicolon after the (#,###0) and my dash, so the pattern looked like this:
#,##0;(#,##0);-
The background color then filled in all cells, including the ones with a dash. If you need spaces in the cells instead of dashes, you would put the semicolon alone without an additional character.
If you only have one pattern like this #,##0, it worked if I added two semicolons and then nothing if I wanted a space and a dash if i wanted a dash.
