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
payload.append(
                {
                    'dag_id': dag_id,
                    'active_dag_run': active_dag_runs,
                    'max_active_runs': max_active_runs,
                }
            )
        return wwwutils.json_response(payload)
    def _mark_dagrun_state_as_failed(self, dag_id, execution_date, confirmed, origin):
        if not execution_date:
            flash('Invalid execution date', 'error')
            return redirect(origin)
        execution_date = timezone.parse(execution_date)
        dag = current_app.dag_bag.get_dag(dag_id)
        if not dag:
            flash(f'Cannot find DAG: {dag_id}', 'error')
            return redirect(origin)
        new_dag_state = set_dag_run_state_to_failed(dag, execution_date, commit=confirmed)
        if confirmed:
            flash(f'Marked failed on {len(new_dag_state)} task instances')
            return redirect(origin)
        else:
            details = '\n'.join(str(t) for t in new_dag_state)
            response = self.render_template(
                'airflow/confirm.html',
                message="Here's the list of task instances you are about to mark as failed",
                details=details,
            )
            return response
    def _mark_dagrun_state_as_success(self, dag_id, execution_date, confirmed, origin):