Default values for constant sum matrix questions

I am using a constant sum matrix question with default values set to 0. However, sometimes survey respondents clear out the 0s as they enter their response. As a result, when the data is downloaded, some of the 0s show as empty fields. This of course impacts the averages that we calculate for each field.
Is there a way to either force the 0s to remain place or input 0s for all empty fields once the question is submitted?