To clear out or empty the values in person or group columns, how you do it depends on if the column is single-value or multi-value.
For single-value person columns, all you have to do is set the value to -1.
It requires an integer value, so null or empty string will give you an error: Cannot convert a primitive value to the expected type ‘Edm.Int32′.
For multi-value person columns, you have to set the results value to an empty array:
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" ]{'__metadata':{'type':'SP.Data.TestListListItem'}, 
'Title':'My title', 
'SinglePersonColumnId':-1, 
'MultiPersonColumnId':{'results':[0]}} 
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