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
"""Set task instance state."""
        try:
            count = len(tis)
            for ti in tis:
                ti.set_state(target_state, session)
            session.commit()
            flash(f"{count} task instances were set to '{target_state}'")
        except Exception:  
            flash('Failed to set state', 'error')
    @action('set_running', "Set state to 'running'", '', single=False)
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
        ]
    )
    def action_set_running(self, tis):
        """Set state to 'running'"""
        self.set_task_instance_state(tis, State.RUNNING)
        self.update_redirect()
        return redirect(self.get_redirect())
    @action('set_failed', "Set state to 'failed'", '', single=False)
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
        ]
    )
    def action_set_failed(self, tis):