Are you ID's really VARCHAR(50)?
You can set the value to NULL instead of empty string. You can use an IF..ELSE control structure. You can use a CASE expression. You can use the NULLIF function.. etc.. It really depends on how you are calling "it." Are you calling a stored procedure? If so, you can make the default value NULL and then not specify that parameter, etc.. Lots of options.
Here is a sample using the NULLIF function:
SET Product_Id = 'PCCV'
,Fund_Id = NULLIF(@NewFund_Id, '')
WHERE Product_Id = @Product_Id