Python version: 3.7.9

Airflow version: 2.1.0

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/local/lib/python3.7/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.7/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/local/lib/python3.7/site-packages/airflow/www/auth.py", line 34, in decorated
    return func(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www/decorators.py", line 97, in view_func
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www/decorators.py", line 60, in wrapper
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/airflow/www/views.py", line 4003, in list
    if SerializedDagModel.get_max_last_updated_datetime() > self.last_refresh:
TypeError: '>' not supported between instances of 'NoneType' and 'datetime.datetime'

**What you expected to happen**:
See the dags dependencies

**What do you think went wrong?**
It's happen only if I don't have any dag yet.

**How to reproduce it**:
With any dag created click in 
menu -> browser -> dag-dependencies
<!---

airflow/www/views.py
y_points = defaultdict(list)
        x_points = defaultdict(list)
        cumulative_y = defaultdict(list)
        task_instances = dag.get_task_instances(start_date=min_date, end_date=base_date)
        ti_fails = (
            session.query(TaskFail)
            .filter(
                TaskFail.dag_id == dag.dag_id,
                TaskFail.execution_date >= min_date,
                TaskFail.execution_date <= base_date,
                TaskFail.task_id.in_([t.task_id for t in dag.tasks]),
            )
            .all()
        )
        fails_totals = defaultdict(int)
        for failed_task_instance in ti_fails:
            dict_key = (
                failed_task_instance.dag_id,
                failed_task_instance.task_id,
                failed_task_instance.execution_date,
            )
            if failed_task_instance.duration:
                fails_totals[dict_key] += failed_task_instance.duration
        for task_instance in task_instances:
            if task_instance.duration:
                date_time = wwwutils.epoch(task_instance.execution_date)
                x_points[task_instance.task_id].append(date_time)
                y_points[task_instance.task_id].append(float(task_instance.duration))
                fails_dict_key = (task_instance.dag_id, task_instance.task_id, task_instance.execution_date)
                fails_total = fails_totals[fails_dict_key]