Is null really null ?

I have a HANA table with a field defined like:

"BUDGET_ASSIGNED" NVARCHAR(1) DEFAULT ' ' NOT NULL ,

So when I see that I assume that I can not put a null value to that field, right?

However I run this update

update "my_schema"."my_table" set BUDGET_ASSIGNED = 'C' where BUDGET_ASSIGNED is null;

no records get updated

I run this query:

select BUDGET_ASSIGNED, length(BUDGET_ASSIGNED) from "my_schema"."my_table" and I get:

so obviously I have some null records

when I run this update:

update "my_schema"."my_table" set BUDGET_ASSIGNED = 'C' where BUDGET_ASSIGNED '';

I get records updated

So ... I'm not really getting that "not null" definition.

Mike