Hi Mike,
This can be controlled with an environment variable.
See:
suma -environment
for a list of all suma-related ENVs. The output shows information, default values and your current settings (if you have a ~/.sumarc). You can use that output to update your own ~/.sumarc without loosing your own preferences.
The variable of interest to you will be: SUMA_MaskZero and it should be set to NO to allow you to see the zeros.
cheers,
z