Return Zero instead of null

I have a function below which works just fine but I would like to return 0 (Zero) instead of null.  I had a go here as you can see, but it does not work this way.

ALTER FUNCTION [dbo].[ufn_GetWorkedHoursWeekEnd]
(
@UserName    varchar(50),
@Month    varchar(50),
@Year int
 )
RETURNS decimal(9,2)
BEGIN
DECLARE @WorkedHours decimal(9,2) =0
Select @WorkedHours = (SELECT Sum([TIME]) as Total FROM [Timesheets].[dbo].[TIMESHEET]
                                    WHERE [IdVolNow] = @Username
                                    AND Year([Date]) = @Year
                                    AND DATENAME(month, [Date]) = @Month
                                    AND (datename(dw,[TIMESHEET].[Date]) = 'Saturday' OR
                                    datename(dw,[TIMESHEET].[Date]) = 'Sunday'))

IF @WorkedHours = Null
BEGIN
      SET @WorkedHours = 0
END

Return @WorkedHours

END