Merge pull request #6385 from entur/fix_bug_book_when_is_null

Fix `bookWhen` field is `null` in the Transmodel API

application/src/main/java/org/opentripplanner/apis/transmodel/model/timetable/BookingArrangementType.java

@@ -109,7 +109,7 @@ public static GraphQLObjectType create() {
           .type(EnumTypes.PURCHASE_WHEN)
           .dataFetcher(environment -> {
             BookingInfo bookingInfo = bookingInfo(environment);
-            if (bookingInfo.getMinimumBookingNotice() != null) {
+            if (bookingInfo.getMinimumBookingNotice().isPresent()) {
               return null;
             }
             BookingTime latestBookingTime = bookingInfo.getLatestBookingTime();