If you have a not null constraint, you have to provide a non-null value on insert/update:
create table t (
c1 int default null not null
);
insert into t values ( null );
ORA-01400: cannot insert NULL into ("CHRIS"."T"."C1")
insert into t values ( 1 );
1 row inserted.
update t set c1 = null;
ORA-01407: cannot update ("CHRIS"."T"."C1") to NULL