Fairly sure this is a "your (or your counterparty's) bank is broken" thing, not a "SWIFT is broken" thing. ISO 15022 (as used here) and ISO 20022 (as used by fancy modern banks who are willing to get onboard with a message format a mere 20 years old) will handle the string 'null' just fine. ISO 15022 doesn't, IIRC, really have a way to represent null the concept at all; you just wouldn't provide the relevant field.
At a guess, the message parsing by the receiving bank is likely to be broken.