solution : ISNULL ( check_expression , replacement_value )

declare @a float;

set @a  = ISNULL((select SUM(CONVERT(FLOAT,quentity)) as qty  from Cutting 

                   WHERE jobno = '1' and contractorname = 'submit'),0)

-- here when quentity is null then it is return "0" else your calculate value;

Thank You

[Sumit Singh Shekhawat]