Sentry Issue: [POSTHOG-37H](https://sentry.io/organizations/posthog/issues/2511149053/?referrer=github_integration)

```
AttributeError: 'NoneType' object has no attribute 'strftime'
(3 additional frame(s) were not displayed)
...
  File "ee/clickhouse/views/insights.py", line 90, in calculate_funnel
    "result": ClickhouseFunnelTrends(team=team, filter=filter, funnel_order_class=funnel_order_class).run()
  File "ee/clickhouse/queries/funnels/base.py", line 48, in run
    results = self._exec_query()
  File "ee/clickhouse/queries/funnels/funnel_trends.py", line 61, in _exec_query
    return self._summarize_data(super()._exec_query())
  File "ee/clickhouse/queries/funnels/base.py", line 118, in _exec_query
    query = self.get_query()
  File "ee/clickhouse/queries/funnels/funnel_trends.py", line 112, in get_query
    {interval_method}(toDateTime('{self._filter.date_from.strftime(TIMESTAMP_FORMAT)}') + number * {seconds_in_interval}) AS entrance_period_start
```
ee/clickhouse/queries/funnels/test/test_funnel_trends.py
filter = Filter(
            data={
                "insight": INSIGHT_FUNNELS,
                "display": TRENDS_LINEAR,
                "interval": "week",
                "date_from": "2021-05-01 00:00:00",
                "date_to": "2021-05-07 00:00:00",
                "funnel_window_days": 7,
                "events": [
                    {"id": "step one", "order": 0},
                    {"id": "step two", "order": 1},
                    {"id": "step three", "order": 2},
                ],
            }
        )
        _create_person(distinct_ids=["user_one"], team=self.team)
        
        _create_event(event="step one", distinct_id="user_one", team=self.team, timestamp="2021-05-01 00:00:00")
        _create_event(event="step two", distinct_id="user_one", team=self.team, timestamp="2021-05-01 01:00:00")
        _create_event(event="step three", distinct_id="user_one", team=self.team, timestamp="2021-05-01 02:00:00")
        results = ClickhouseFunnelTrends(filter, self.team, ClickhouseFunnel)._exec_query()
        persons, _ = self._get_people_at_step(filter, "2021-04-25 00:00:00", False)
        self.assertEqual(2, len(results))
        self.assertEqual(
            [person["distinct_ids"] for person in persons], [["user_one"]],
        )
    def test_month_interval(self):