Apply @NotNull to start and end date

It was not applied to end date. Besides it must be applied to
IntraDayDate properties instead of Date properties because the Date
properties are generated from the IntraDayDate properties.

FEA: ItEr64OTS03PlanificacionHaciaAtras

navalplanner-business/src/main/java/org/navalplanner/business/planner/entities/TaskElement.java

@@ -232,12 +232,12 @@ public abstract class TaskElement extends BaseEntity {
         return Collections.unmodifiableSet(dependenciesWithThisDestination);
     }
 
-    @NotNull
     public Date getStartDate() {
         return startDate != null ? startDate.getDate().toDateTimeAtStartOfDay()
                 .toDate() : null;
     }
 
+    @NotNull
     public IntraDayDate getIntraDayStartDate() {
         return startDate;
     }
@@ -304,6 +304,7 @@ public abstract class TaskElement extends BaseEntity {
                 + "Please detect the caller and fix it";
     }
 
+    @NotNull
     public IntraDayDate getIntraDayEndDate() {
         return endDate;
     }