There isn't one unfortunately... C2 doesn't have any referencing for "null" values. For official plugins everything is forced to Numbers, Strings or Boolean. nulls are usually returned as int(0) and then converted to what ever you are setting to.
You have a var myString that you set from a function return and some how that return value is null (shouldn't be possible unless using a 3rd party plugin) this would be the resulting outcome.
var myString = ""; (defined as type string)
myString = myfunction.returnValue; (set myString from function return value. if null it will return int(0))
myString equals "0" (stringified version of int(0))
Based on looking at the plugins this appears to be by design to either prevent system errors or non-programmers from making mistakes. We just have to work around this limitation.