You could always map over the array and set the undef values to '', I would personally suggest keeping the database pristine. I would use nulls if it increases data clarity for a more ugly solution in the perl base. '' would violate a unique constraint where as multiple nulls woulden't for instance. Nulls are also open to the database implimentation, and can be optimized away very easily, an empty string however is kept as just as that, rather than a small pointer to null.
or a simple join ",", grep defined, @row;