Exit early if 0 rows detected.

In !1 (merged) we forgot to account for when there is no data to merge!

SELECT DISTINCT date_trunc('HOUR', revision.rev_dt) as modified_hours
FROM  event.rc1_mediawiki_page_content_change
WHERE year = 2023
  AND month = 5
  AND day = 1
  AND hour = 3
ORDER BY modified_hours DESC -- we are ORing this later so let's get it in likelyhood order now

Result:

+--------------+
|modified_hours|
+--------------+
+--------------+

This MR fixes this.