PLANNER-1540 Clarify "potential" NPE

optaplanner-examples/src/main/java/org/optaplanner/examples/flightcrewscheduling/persistence/FlightCrewSchedulingXlsxFileIO.java

@@ -602,6 +602,7 @@ private Map<Integer, List<FlightAssignment>> extractHourToAssignmentListMap(
                     } else {
                         previousArrivalHour = Math.max(previousArrivalHour, arrivalHour);
                     }
+                    // The list is never null; gets initialized in the first loop due to previousArrivalHour = -1
                     previousFlightAssignmentList.add(flightAssignment);
                 }
             }