When creating a table with multiple Primary Key , then inserting into columns that are primary key with one null value or all of them was null , the records inserting without error ! but that is a constraint violation! example :

create table if not exists tbl2 (
name TEXT ,
last_name TEXT ,
PRIMARY KEY (name,last_name)
);
insert into tbl2 values('s','g');
// inserting above value again throws error
// but
insert into tbl2 values('s',null);
// inserting above value again it doesn't throws error
// or below insert
insert into tbl2 values(null,null)