Hello and welcome to the Postman community.
One quick question about this, are you looking to have the actual value of
null in the data or are you just looking for it to be blank?
Unfortunately it seem that Postman does not allow for null values to be used in .csv files anymore. There is a workaround that you might be able to use though.
You are able to upload data files in both csv and json format. I was able to verify that the json format does work for the null use case.
More information can be found at the below link.
I will use your method!