Default value in calculated field

I've got an SQL query that works great, unless there are 0 records found. Is there any way to make "ttlTotal" = 0? Currently it returns NULL when there are no records found.

SELECT
  UserName,
  SUM(ifNull(Day1Reg, 0) +
      ifNull(Day2Reg, 0) +
      ifNull(Day3Reg, 0) +
      ifNull(Day4Reg, 0) +
      ifNull(Day5Reg, 0) +
      ifNull(Day6Reg, 0) +
      ifNull(Day7Reg, 0) +
      ifNull(Day1OT, 0) +
      ifNull(Day2OT, 0) +
      ifNull(Day3OT, 0) +
      ifNull(Day4OT, 0) +
      ifNull(Day5OT, 0) +
      ifNull(Day6OT, 0) +
      ifNull(Day7OT, 0)) as ttlTotal
FROM weeks
WHERE RecNum = 1
AND
  (Day1Reg IS NOT NULL OR
   Day1OT IS NOT NULL OR
   Day2Reg IS NOT NULL OR
   Day2OT IS NOT NULL OR
   Day3Reg IS NOT NULL OR
   Day3OT IS NOT NULL OR
   Day4Reg IS NOT NULL OR
   Day4OT IS NOT NULL OR
   Day5Reg IS NOT NULL OR
   Day5OT IS NOT NULL OR
   Day6Reg IS NOT NULL OR
   Day6OT IS NOT NULL OR
   Day7Reg IS NOT NULL OR
   Day7OT IS NOT NULL)
GROUP BY UserName