I know, it's bad. I agree that it should've stayed NULL. But if you really really really think you have an acceptable reason to do something like this (and you probably don't), the least you can do is use a non-trivial token.
replyIf I was really desperate and could only return a valid string, I would probably use a message and add a random component to avoid ever matching two errors. Like SQL, where null != null.
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