quick fix task end date can not be null + test data

src/main/java/ch/zhaw/gartenverwaltung/TaskFormularController.java

@@ -135,6 +135,7 @@ public class TaskFormularController implements Initializable {
         });
 
         button.disableProperty().bind(start_datePicker.valueProperty().isNull()
+                .or(end_datePicker.valueProperty().isNull())
                 .or(taskName_field.textProperty().isEmpty())
                 .or(description_area.textProperty().isEmpty()));
     }

src/main/java/ch/zhaw/gartenverwaltung/types/TaskTemplate.java

@@ -42,7 +42,7 @@ public class TaskTemplate {
     }
 
     public Task generateTask(LocalDate realStartDate, long cropId) {
-        LocalDate endDate = relativeEndDate != null ? realStartDate.plusDays(relativeEndDate) : null;
+        LocalDate endDate = relativeEndDate != null ? realStartDate.plusDays(relativeEndDate) : realStartDate.plusDays(0);
 
         if (interval == null) {
             this.interval = 0;