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There's a lot of people on Earth with only one name, it's a common occurrence in Indonesia. So to answer your question: yes I think she has only Cher on her passport.
You are still exhibiting that harmful attitude. I know plenty of British people who are not so daft - most of them in fact, so I don't believe your supposition that it is a common attitude. And even if it was, that's never a justification.
"I want everyone to call me ABC123!"
"No, will you ever knock it off Bob"
The majority of people outside the coastal tech bubble would simply tell anyone making this kind of demand to catch themselves on. There's not enough of the community reining in this kind of hubristic nonsense.
Also "Null" is a perfectly pronounceable and fine name. A brief search reveals several people with that name on Facebook, so it's clearly not abnormal, nor impossible to design a name field to handle it.