It’s not possible to set an enum type with a null value
This is default behavior of Base Enumeration. By default if user doesn’t choose a value the index 0 is defaulted for new record.
If you want to achieve what you are looking for, you may add a third element in Base Enum, set third element as “Not Specified”. Make sure to keep your element “Not Specified” with Int Value 0 so it will be defaulted as default vale.