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
"""Pre update hook."""
        item.execution_date = timezone.make_aware(item.execution_date)
        item.value = XCom.serialize_value(item.value)
def lazy_add_provider_discovered_options_to_connection_form():
    """Adds provider-discovered connection parameters as late as possible"""
    def _get_connection_types() -> List[Tuple[str, str]]:
        """Returns connection types available."""
        _connection_types = [
            ('fs', 'File (path)'),
            ('mesos_framework-id', 'Mesos Framework ID'),
        ]
        providers_manager = ProvidersManager()
        for connection_type, (_, _, _, hook_name) in providers_manager.hooks.items():
            _connection_types.append((connection_type, hook_name))
        return _connection_types
    ConnectionForm.conn_type = SelectField(
        lazy_gettext('Conn Type'),
        choices=sorted(_get_connection_types(), key=itemgetter(1)),
        widget=Select2Widget(),
        validators=[InputRequired()],
        description="""
            Conn Type missing?
            Make sure you've installed the corresponding Airflow Provider Package.
        """,
    )
    for key, value in ProvidersManager().connection_form_widgets.items():
        setattr(ConnectionForm, key, value.field)
class ConnectionFormWidget(FormWidget):