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/funnel_trends.py
serialized_result = {
                "timestamp": period_row[0],
                "reached_from_step_count": period_row[1],
                "reached_to_step_count": period_row[2],
                "conversion_rate": period_row[3],
                "is_period_final": self._is_period_final(period_row[0]),
            }
            if breakdown_clause:
                serialized_result.update(
                    {
                        "breakdown_value": period_row[-1]
                        if isinstance(period_row[-1], str)
                        else Cohort.objects.get(pk=period_row[-1]).name
                    }
                )
            summary.append(serialized_result)
        return summary
    def _format_results(self, summary):
        if self._filter.breakdown:
            grouper = lambda row: row["breakdown_value"]
            sorted_data = sorted(summary, key=grouper)
            final_res = []
            for key, value in groupby(sorted_data, grouper):
                breakdown_res = self._format_single_summary(list(value))
                final_res.append({**breakdown_res, "breakdown_value": key})
            return final_res
        else:
            res = self._format_single_summary(summary)
            return [res]
    def _format_single_summary(self, summary):