There is no refund. It is unlikely that there will be, it is about a lot of companies.  It is not the entire railway, the main train transporter does not strike. 

It looks like TS travels with a regional operator only. Those actually do offer a compensation during strikes based on the Algemene voorwaarden Stads- en Streekvervoer.

For strikes with a duration of multiple days you'll be eligible for a refund of 1/365th of the yearly subscription costs for each day of the strikes. You'll have to contact the regional operator for that however and not NS.