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I don't see how it differs from the default.
I agree. It's also unused. In fact, I believe [TreatUndefinedAs] as a whole can go. Specifically, for the values: "Missing" is obsoleted by making that the default behavior. "Null" is pointless as this bug says "EmptyString" can be replaced by just making the argument optional and giving it a default value of "".
As I filed a bug for a weak ago, yes... *** This bug has been marked as a duplicate of bug 23701 ***