Optional variable/parameter in SSIS
You may be interested in a standard approach to solve this problem. It is something called short circuit. To illustrate it, here's the code:

DECLARE @chooseDate datetime2
--SET @chooseDate = '1966-04-08'
SET @chooseDate = NULL
SELECT *
FROM DimCustomer
WHERE @chooseDate IS NULL OR BirthDate = @chooseDate


This way you can explicitly set variable to NULL, do not set variable to anything, or specify a value. 

The reason it is called short circuit is that based on the implementation of ANSI SQL this line of code:

WHERE @chooseDate IS NULL OR BirthDate = @chooseDate


has a possibility to evaluate only the first part of the expression (the IS NULL part) thus excluding the need to evaluate the second part, which makes the code slightly faster.