At the moment the documentation for $w.RatingsDisplay.rating states:
This element does not have a value property so you cannot reset the Ratings Display element to "no ratings" this way. The assumption would be that this text is referring to the rating property. If you try to assign undefined or null to the rating property a wix SDK warning is triggered stating the following:To reset the rating display element, restoring it to the empty state, set the value property to nullor undefined.
I would suggest that if the numRatings property is set to zero then there would be no ratings and so the element should be reset accordingly. It would be unusual for there to be any applied ratings if there were none on record ;-)