How to get CASE statement to recognize null ?

[David Gauthier]

This is probably an easy one for someone with experience using CASE, but
intuitively I can't get it.
First... 11.5 on linux.

Example...

    dvdb=# create table foo (col1 varchar, col2 varchar);
    CREATE TABLE
    dvdb=# insert into foo (col1,col2) values ('a','x'), (null,'y');
    INSERT 0 2
    dvdb=# select * from foo;
     col1 | col2
    ------+------
     a    | x
          | y
    (2 rows)

    dvdb=# select
      CASE col1
        WHEN null THEN 'z'
        ELSE col1
       END as col1,
       col2
     from foo;

     col1 | col2
    ------+------
     a    | x
          | y
    (2 rows)

For the 2nd rec, col1 is null, so why wasn't it changed to 'z' ?

    dvdb=# select
      CASE col1
        WHEN null THEN col1
        ELSE 'z'
      END as col1,
      col2
    from foo;
     col1 | col2
    ------+------
     z    | x
     z    | y
(2 rows)

The 'a' in col1 of the first rec got clobbered as if it matched "null".

Obviously the problem has to do with how I'm specifying "null".  But what's
the right way to do that ?

Thanks in Advance