AX does not have a null value concept. Instead the following values are considered "not entered" by defintion:
string: blank
int and int64: 0 (zero)
enum: 0 (typically the first value)
date: 01\01\1900 (displays as blank)
For new base enums make a blank zero enum value (by convention name it None). This will make the use of mandatory fields possible for this enum type.