Foreign Keys

I am able to enter a null in a foreign key field in one table(table2)
without the foreign key relationship (table1.t1_id) validating the value.
Why is this?

Below are 2 table's scripts:

create table table1 (
    t1_id         VARCHAR(32) not null,
    t1_desc         VARCHAR(32)      null,
constraint table1_pkey primary key (t1_id) )  ;


create table table2 (
    t2_id        VARCHAR(32) not null,
    t1_id         VARCHAR(32) null,
                t2_desc                     VARCHAR(32) null,
  constraint table2_pkey primary key (t2_id) )  ;

create index table2_t1_id
    on table2 ( t1_id )  ;

alter table table2
    add constraint table1_table2_t1_id_FK1
    foreign key ( t1_id )
    references table1 ( t1_id )
    ON DELETE RESTRICT ON UPDATE CASCADE ;

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