Slicer with Blank value

Hello All, 

I have 3 tables like this 

and its relation like this:

I am creating a table  viz in Power BI with Stu ID and Name, and I want o filter based on Dept,

like this: 

How can I add Blank selection in Dept Filter, so that I get list of all Student who doesnot have Department.

Unfortunately I cannot modify Stu-Dept table. 

Thanks in Advance!