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
)
                    for ti in tis
                )
            )
        )
        tasks = []
        for ti in tis:
            
            
            
            try_count = ti.prev_attempted_tries if ti.prev_attempted_tries != 0 else ti.try_number
            task_dict = alchemy_to_dict(ti)
            task_dict['end_date'] = task_dict['end_date'] or timezone.utcnow()
            task_dict['extraLinks'] = dag.get_task(ti.task_id).extra_links
            task_dict['try_number'] = try_count
            tasks.append(task_dict)
        tf_count = 0
        try_count = 1
        prev_task_id = ""
        for failed_task_instance in ti_fails:
            if tf_count != 0 and failed_task_instance.task_id == prev_task_id:
                try_count += 1
            else:
                try_count = 1
            prev_task_id = failed_task_instance.task_id
            tf_count += 1
            task = dag.get_task(failed_task_instance.task_id)
            task_dict = alchemy_to_dict(failed_task_instance)
            end_date = task_dict['end_date'] or timezone.utcnow()
            task_dict['end_date'] = end_date