Fix for special_price active check

This is a backport for a vuestorefront/vue-storefront#2838 fix from develop branch

Author: pkarw

src/lib/taxcalc.js

@@ -1,7 +1,7 @@
 function isSpecialPriceActive(fromDate, toDate) {
   const now = new Date()
-  fromDate = new Date(fromDate) || false
-  toDate = new Date(toDate) || false
+  fromDate = fromDate ? new Date(fromDate) : false
+  toDate = toDate ? new Date(toDate) : false
 
   if (!fromDate && !toDate) {
     return true