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/models/serialized_dag.py
"""
        Get the latest DAG version for a given DAG ID.
        :param dag_id: DAG ID
        :type dag_id: str
        :param session: ORM Session
        :type session: Session
        :return: DAG Hash
        :rtype: str | None
        """
        return session.query(cls.dag_hash).filter(cls.dag_id == dag_id).scalar()
    @classmethod
    @provide_session
    def get_dag_dependencies(cls, session: Session = None) -> Dict[str, List['DagDependency']]:
        """
        Get the dependencies between DAGs
        :param session: ORM Session
        :type session: Session
        """
        if session.bind.dialect.name in ["sqlite", "mysql"]:
            query = session.query(cls.dag_id, func.json_extract(cls.data, "$.dag.dag_dependencies"))
            iterator = ((dag_id, json.loads(deps_data) if deps_data else []) for dag_id, deps_data in query)
        elif session.bind.dialect.name == "mssql":
            query = session.query(cls.dag_id, func.json_query(cls.data, "$.dag.dag_dependencies"))
            iterator = ((dag_id, json.loads(deps_data) if deps_data else []) for dag_id, deps_data in query)
        else:
            iterator = session.query(cls.dag_id, func.json_extract_path(cls.data, "dag", "dag_dependencies"))
        return {dag_id: [DagDependency(**d) for d in (deps_data or [])] for dag_id, deps_data in iterator}