Correctly split states which contain a mode for a single State

When traversing *Link edges `backMode = null`, which in most cases
doesn't cause problems. When splitting Flex states, there is only a
single `BUS` state, which the logic skipped.

The last-seen mode is tracked, instead of the previous mode, which
correct the check for mode changes.

src/main/java/org/opentripplanner/routing/algorithm/mapping/GraphPathToItineraryMapper.java

@@ -202,13 +202,18 @@ private static State[][] sliceStates(State[] states) {
         int[] legIndexPairs = {0, states.length - 1};
         List<int[]> legsIndexes = new ArrayList<int[]>();
 
+        TraverseMode lastMode = null;
         for (int i = 1; i < states.length - 1; i++) {
             var backState = states[i];
             var forwardState = states[i + 1];
             var backMode = backState.getBackMode();
             var forwardMode = forwardState.getBackMode();
 
-            var modeChange = backMode != forwardMode && backMode != null && forwardMode != null;
+            if (backMode != null) {
+                lastMode = backMode;
+            }
+
+            var modeChange = lastMode != forwardMode && lastMode != null && forwardMode != null;
             var parkingChange = backState.isVehicleParked() != forwardState.isVehicleParked();
             var rentalChange = isRentalPickUp(backState) || isRentalDropOff(backState);