NULL in foreign key column(s)
You are quite right. I have created a small example at Fiddle

create table p
( a int not null
, b int
, unique (a,b)
);

create table c
( a int not null
, b int
, constraint fk_p foreign key (a,b)
                  references p (a,b)
                      on delete cascade
                      on update cascade
);

-- insert data in parent:
insert into p (a,b) values (1,null);  -- ok
insert into p (a,b) values (1,null);  -- also ok. The constraint evaluates to null,
                                      -- which is ok. The rule is that it must not 
                                      -- evaluate to false

-- insert data in child
insert into c (a,b) values (1,null);  -- ok
insert into c (a,b) values (2,null);  -- also ok, foreign key evaluates to null. 
                                      -- So despite there is no 2 in
                                      -- parent this is ok

-- remove data from parent
delete from p;

-- all rows present in child
select * from c;

a   b
1   null
2   null 


Null makes even trivial things a bit tricky to grasp. I would not recommend candidate keys that include nulls.