I need to isolate a string result
which contains a null value.
The idea is to recognize when a string sentence contains a value that is null.

WITH test_data (text_value) AS (
  SELECT 
    'null,test' 
  FROM 
    DUAL 
  UNION ALL 
  SELECT 
    '12345678' 
  FROM 
    DUAL
) 
SELECT 
  td.text_value, 
  CASE WHEN REGEXP_LIKE(NULL, '.') THEN 'Y' ELSE 'N' END AS VALID 
   FROM 
     test_data td

The expected result is to isolate a result that has a null value

null,test N