Hide 0 value in SUM calculations

I have a simple sheet with additions, it looks like this:

B column is for the sum of the values on the right, and if there is no value on a row, it shows 0, but let's say I want it to say 'None' instead, or just be empty if there is no value to sum. B row functions are currently like this:

=SUM(C7:M7)

How can I do it?