Oops. Wer beat me. Don't forget how to OR rules together mate.
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Set up an Expression Column in any audio View and put in as the Expression;
[Rating,0]
That shows the raw value saved in the field. I think you will find that for anything unrated, the contents will be blank. An empty field. [Rating] will show a formatted value, such as "? Stars" for blank value, or a number 1-5 for those with Ratings.
In a Search using [Rating]=<1 will work correctly, showing blank values, as blank is evaluated as zero. See
https://wiki.jriver.com/index.php/Search_Language#Comparison_OperatorsMaybe in a Smartlist you will need to use the expression;
[Ratiing]=[]
Which means Rating equals blank... Yep, that works.
In MC, the Search and Expression languages work slightly differently.
EDIT: PS. Entering a value of 0 into the Rating field actually clears the field, from memory.