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.
https://learning.getpostman.com/docs/postman/collection_runs/working_with_data_files/
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I will use your method!
There is ANOTHER workaround I just found.
If you are set on using csv you can add an additional (unused) column/variable at the end and leave it blank.
You then can just leave a blank for any column you need to leave NULL.
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I have a column call description which is blank and the in the json file is states null. I did create a empty column, but no luck