What is the target application for the exported CSV file?
If the exported column is going to be interpreted as numeric when imported to the target application what will that application do with blanks or specific NULL outputs?
Or to look at is another way - an alternative approach to consider is, like Stephen's suggestion, to create a calculated field and export that instead of the "real" field but just make the field Character. That way if it present in Monarch as blank I would expect it to be exported as blank. However the target application may do something else with it when assessing it for ingestion. Hence the question.