Oracle 21c

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create table t (a char(1));
insert into t(a) values (NULL);
1 rows affected

insert into t(a) values ('a');
1 rows affected

insert into t(a) values ('');
1 rows affected

select a from t

A

null

a

null

select a from t where a is null

A

null

null

A
null
a
null

A
null
null