[Koha] How in koha

[Achamie Aynalem]

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?