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I'm not quite following exactly what you're trying to do, but you may just need to type "NULL" into the Change Data Value flow step instead of leaving it blank..
To prevent this issue altogether, I'd suggest hiding the State field in a Marketo form unless the Country selected is United States or Canada. You can do that via the visibility rules in your form.
You are correct, using NULL in the flow step will take care of it. Leaving the flow step blank or using null won't work as needed.