Using parm in where clause OR

I want to have a parameter that can be either passed in or null.  If it is passed in I want to use it in the where clause to filter the results.  But if it is not passed in I want to show all the business ID's in the Customer table that are joined to the Business Table AND all the Customers that have null Business_Id's


@Business_ID  INT= NULL

Select * FROM CUSTOMER
OUTER JOIN BUSINESS
on Customer.CUSTOMER.Business_ID = BUSINESS.Business_ID
WHERE
CUSTOMER.Business_ID =
CASE
@Business_ID is NOT NULL  --- A parm is passed in
THEN
       @BUSINESS_ID --- use the value of the parm
ELSE
       BUSINESS.Business_ID OR NULL -- problem here
END


I can get most of this to work except for the situation where the Customer Business_ID is Null