[Koha] How in koha

I automated a library using koha 3.12. In my system, I created an -extended patron attribute- called <ID> that holds students' ID Number and checked to be Unique identifier. But while I insert or import patrons from a .csv file; **when I assign value for The <patron attribute> field in the .Csv (like ID:xxxx), it works well so prevents entering different patrons with similar ID (well). **when entering a patron with no value for the ID, it simply enters the patron so entering of so many patrons with null value for their ID (failed). so, how to make koha prevent the entrance of null value for a "unique patron attribute" like ID in my case?