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
nodes = []
        edges = []
        for dag, dependencies in SerializedDagModel.get_dag_dependencies().items():
            dag_node_id = f"dag:{dag}"
            nodes.append(self._node_dict(dag_node_id, dag, "dag"))
            for dep in dependencies:
                nodes.append(self._node_dict(dep.node_id, dep.dependency_id, dep.dependency_type))
                edges.extend(
                    [
                        {"u": f"dag:{dep.source}", "v": dep.node_id},
                        {"u": dep.node_id, "v": f"dag:{dep.target}"},
                    ]
                )
        self.nodes = nodes
        self.edges = edges
    @staticmethod
    def _node_dict(node_id, label, node_class):
        return {
            "id": node_id,
            "value": {"label": label, "rx": 5, "ry": 5, "class": node_class},
        }
class CustomPermissionModelView(PermissionModelView):