If you want a query which returns rows in column containing NULL or empty space then the using NULL alone in where clause will not return empty space rows. So the query should be altered as below

Actual Query

 SELECT COUNT (*) 
   FROM TABLE 
  WHERE COL_NAME IS NULL;

Altered Query

  SELECT COUNT (*) 
    FROM TABLE 
   WHERE trim(COL_NAME) IS NULL

(or)

Altered Query

  SELECT COUNT (*) 
    FROM TABLE  
    WHERE COL_NAME IS NULL OR 
          LENGTH(TRIM (COL_NAME)) = 0