Operation if one value is potentially empty

I’m trying to add together two values which then save to a session storage variable. There is a potential that one of the values is empty. If both values are not null, this works fine with the expected outcome e.g. value1 (1) + value2 (1) saves to the variable as 2. If value1(1) + value2 (empty) the variable isn’t populated.

Is there a way to define ‘not empty’ in data formatting?

Thanks