Return '0' when no results found
===

Is there a way to modify this query to return '0' in the Grade column if no records are found? Right now when nothing is found I get a blank row.

    SELECT (CASE ScanName WHEN 'APP' THEN 'Network System' END) AS System, 
    (sum(CASE severity WHEN 1 THEN 10 WHEN 2 THEN 9 WHEN 3 THEN 6 WHEN 4 THEN 3    
    END)/COUNT(severity )) AS Score,
    (sum(CASE severity WHEN 1 THEN 10 WHEN 2 THEN 9 WHEN 3 THEN 6 WHEN 4 THEN 3   
    END)/COUNT(severity )) AS Grade 

    FROM missing_internal

    WHERE ScanName like '%APP%'

    and LAST_UPDATE >= SUBDATE((SELECT MAX(LAST_UPDATE) from missing_internal), INTERVAL 1 day)
    HAVING System IS NOT NULL  
    ORDER BY LAST_UPDATE DESC LIMIT 1

Score and Grade are the same on purpose as I am using a separate script to apply a letter grade to the score.