Well, I would use an IF..ELSE Statement like this
SELECT CASE WHEN <br>
(SELECT
unix_timestamp(now()) - unix_timestamp(dateTime)
FROM action_table
WHERE action='aaa'
ORDER BY dateTime DESC
LIMIT 1) > 0
THEN
(SELECT
unix_timestamp(now()) - unix_timestamp(dateTime)
FROM action_table
WHERE action='aaa'
ORDER BY dateTime DESC
LIMIT 1)
ELSE
'no result'
END;
Here is a working example on sqlfiddle.