SQL & PL/SQL

Hello Everybody,
I am trying to get a count of all null values v/s non null values in a table. I am trying to use the CASE statement to do this.
I have formed the following SQL so far.

SELECT count(Aj.Asset_Hdr_Id),
       CASE Nvl(Aj.Asset_Hdr_Id,
            0)
         WHEN 0 THEN
          'IS NULL'
         ELSE
          'IS NOT NULL'
       END
  FROM Asset_Jt Aj
 GROUP BY Nvl(Aj.Asset_Hdr_Id, 0);

This is not giving me the desired result. My desired result will look like

2089 IS NULL
12340 IS NOT NULL

I am getting a result that looks like this

1	0	IS NULL
2	1	IS NOT NULL
3	1	IS NOT NULL
4	1	IS NOT NULL
5	1	IS NOT NULL