I recently had occasion to do some more work with transform maps in ServiceNow, and I couldn’t remember if I needed to worry about blank cells from my Excel document over-writing data in ServiceNow, so I did some experimenting.
I can confirm that blank cells in the Excel doc being imported and transformed do not overwrite existing data in the relevant field in ServiceNow.
Now it’s documented and I can refer back here the next time I have this particular question.