There's absolutely no justification for the assertion there "should" be "zero" rating. As a rating scale, there's nothing wrong with it starting at 1. It is, in fact, far more sensible than what you suggest. Using stars alone, there's no practical way to distinguish between NULL and 0. And there's no reason to rate something 0 when you can just as easily decide 1 has the exact same meaning. It's also obvious most users will find it perfectly intuitive the way it is. With a 0 rating allowed, we'd be forever explaining the distinction between "0" and "unrated."
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There is nothing wrong with having a rating scale starting at 0.
If a movie is a real stinker 0 is perfectly valid.
null and 0 are different.
0 is a value - by definition - the value between + and - numbers
null is not populated. ie the field in question is empty. ( an invisible character eg "space" is a value and does not meet "null" criteria ).
I can see merit in having a rating system starting at 0 and null.
eg.
a movie is a stinker would be 0.
I haven't given this movie a rating yet would be null
Both should be independently searchable.
Using stars alone, there's no practical way to distinguish between NULL and 0.
perhaps if the movie is unrated "
there is no spoon " err - there are no stars (displayed).
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