CSR-1438

If a customer navigates backwards and clears their email, we send an empty string to save-session which will fail validation.  Pass nulls instead

src/store/index.js

@@ -1945,7 +1945,7 @@ export const actions = {
     },
 
     saveEmail(context, email) {
-        context.commit(storeMutations.UPDATE_CUSTOMER_EMAIL_ADDRESS, email);
+        context.commit(storeMutations.UPDATE_CUSTOMER_EMAIL_ADDRESS, email === "" ? null : email);
     },
 
     saveVin(context, { isSelectedGlassAvailableForVehicle, vehicleInfo }) {