You will have to modify your SP. One option is to change it like
WHERE
Date_Created >= DATEADD (YYYY, - 2, GETDATE()) AND
Date_Created <= GETDATE() AND
((LTrim(RTrim(Customer)) = @customername) OR (@customername IS NULL))
SO when you dont pass any value to @customername it picks up all customers.
EDIT: Also I would change this
Date_Created >= DATEADD (YYYY, - 2, GETDATE())
to this
Date_Created >= DATEADD (YYYY, - 2,dateadd(d, datediff(d, 0, getdate()), 0))