Great work on moving towards updating and improving the web publishing engine with the addition of the data API!
According to RFC 5789, sparse updates of resources should be delivered via PATCH. PUT is for complete replacement of a resource. PATCH will leave columns left out of the payload untouched, while PUT must restore any columns not included in the request back to their default values.
The suggestion here is that you get off on the right foot wrt standards with this new feature and use PATCH for your sparse updates. This will leave PUT available for the possibility of 'resetting' a resource with a partial payload.