If you are performing a 'Custom calculation script' you can test to see if you have the necessary input values and perform the calculation and show a zero value if that is the result of the calculation.
If you are using the short cut calculations, you can use the following script for a "Custom validation scrpt':
// if this field's value (event) is equal to zero set field value to null string
if(event.value == 0) evetn.value = '';
The above code is for forms created in Acrobat using Acrobat's Formtool and not forms created with LiveCycle Designer.