[app_dart] Fix stale tasks timestamp encoding (#2483)

* [app_dart] Fix stale tasks timestamp encoding

* flutterfmt
diff --git a/app_dart/lib/src/request_handlers/scheduler/vacuum_stale_tasks.dart b/app_dart/lib/src/request_handlers/scheduler/vacuum_stale_tasks.dart
index 2594864..852c526 100644
--- a/app_dart/lib/src/request_handlers/scheduler/vacuum_stale_tasks.dart
+++ b/app_dart/lib/src/request_handlers/scheduler/vacuum_stale_tasks.dart
@@ -77,9 +77,8 @@
       }
     }
 
-    final Iterable<Task> inserts = tasksToBeReset
-        .map((Task task) => task..status = Task.statusNew)
-        .map((Task task) => task..createTimestamp = null);
+    final Iterable<Task> inserts =
+        tasksToBeReset.map((Task task) => task..status = Task.statusNew).map((Task task) => task..createTimestamp = 0);
     await datastore.insert(inserts.toList());
   }
 }
diff --git a/app_dart/test/request_handlers/scheduler/vacuum_stale_tasks_test.dart b/app_dart/test/request_handlers/scheduler/vacuum_stale_tasks_test.dart
index 68cf482..3135b79 100644
--- a/app_dart/test/request_handlers/scheduler/vacuum_stale_tasks_test.dart
+++ b/app_dart/test/request_handlers/scheduler/vacuum_stale_tasks_test.dart
@@ -100,7 +100,7 @@
       expect(tasks[0], originalTasks[0]);
       expect(tasks[1], originalTasks[1]);
       expect(tasks[2].status, Task.statusNew);
-      expect(tasks[2].createTimestamp, isNull);
+      expect(tasks[2].createTimestamp, 0);
     });
   });
 }