Force zeros in Excel 2007 when cells blank and keep commas for thousands
 

I am using Excel 2007 for my spreadsheet.

When I clear out the cells for the sales part, I want it to force
zeros as values. When the sales are in the cells, I want to display
with the comma format. For example 12345 should show up as 12,345

I'm thinking I need to push the custom format with #,##0
and something additional to force the zeros, but I don't know what to
do additionally.

Please advise.