fix(tasks): newly assigned `None` in registry (#2381)

tests/test_tasks.py

@@ -80,6 +80,18 @@ async def test_purge_tasks(app: Sanic):
     assert len(app._task_registry) == 0
 
 
+async def test_purge_tasks_with_create_task(app: Sanic):
+    app.add_task(asyncio.create_task(dummy(3)), name="dummy")
+
+    await app.cancel_task("dummy")
+
+    assert len(app._task_registry) == 1
+
+    app.purge_tasks()
+
+    assert len(app._task_registry) == 0
+
+
 def test_shutdown_tasks_on_app_stop():
     class TestSanic(Sanic):
         shutdown_tasks = Mock()