Field Calculations for Merged Shapefile

I have a merged shapefile and I want to use the field calculator to assign a Project # from my "New Data Project #" field to another field (Project #) that already contains valid PJ #'s from a shapefile that existed before the merge. The only catch is I only want to export values from "New Data Project #" if the other value in the field Project # is null. Any advice on a vb code or python code i could use in the field calculator to accomplish this?

Thanks