merge fields to keep choices should be expanded

When deduplicating, it would be great to be able to have more choice in terms of which data we want to keep. 



For example, by default if field is empty and another one filled, it does not give me the choice if I want to fill with empty value or not.



Let's say my contacts have a contract number. Old version of contact is a client with contract number, new version of contact has no client number anymore.



The system does not allow me to select blank value as value of choice. It will take the old contract number that I do not want to keep...