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
timestamp="2021-05-05 00:00:00",
            properties={"$browser": "Chrome"},
        )
        _create_person(
            distinct_ids=["user_three"], team=self.team, properties={"$browser": "Safari"},
        )
        _create_event(
            event="step one",
            distinct_id="user_three",
            team=self.team,
            timestamp="2021-05-03 00:00:00",
            properties={"$browser": "Safari"},
        )
        _create_event(
            event="step two",
            distinct_id="user_three",
            team=self.team,
            timestamp="2021-05-04 00:00:00",
            properties={"$browser": "Safari"},
        )
        _create_event(
            event="step three",
            distinct_id="user_three",
            team=self.team,
            timestamp="2021-05-05 00:00:00",
            properties={"$browser": "Safari"},
        )
        filter = Filter(
            data={
                "insight": INSIGHT_FUNNELS,