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
from posthog.constants import BREAKDOWN
from posthog.models.cohort import Cohort
from posthog.models.filters.filter import Filter
from posthog.models.team import Team
TIMESTAMP_FORMAT = "%Y-%m-%d %H:%M:%S"
HUMAN_READABLE_TIMESTAMP_FORMAT = "%-d-%b-%Y"
class ClickhouseFunnelTrends(ClickhouseFunnelBase):
    """
    ## Funnel trends assumptions
    Funnel trends are a graph of conversion over time – meaning a Y ({conversion_rate}) for each X ({entrance_period}).
    ### What is {entrance_period}?
    A funnel is considered entered by a user when they have performed its first step.
    When that happens, we consider that an entrance of funnel.
    Now, our time series is based on a sequence of {entrance_period}s, each starting at {entrance_period_start}
    and ending _right before the next_ {entrance_period_start}. A person is then counted at most once in each
    {entrance_period}.
    ### What is {conversion_rate}?
    Each time a funnel is entered by a person, they have exactly {funnel_window_interval} {funnel_window_interval_unit} to go
    through the funnel's steps. Later events are just not taken into account.
    For {conversion_rate}, we need to know reference steps: {from_step} and {to_step}.
    By default they are respectively the first and the last steps of the funnel.
    Then for each {entrance_period} we calculate {reached_from_step_count} – the number of persons
    who entered the funnel and reached step {from_step} (along with all the steps leading up to it, if there any).
    Similarly we calculate {reached_to_step_count}, which is the number of persons from {reached_from_step_count}
    who also reached step {to_step} (along with all the steps leading up to it, including of course step {from_step}).
    {conversion_rate} is simply {reached_to_step_count} divided by {reached_from_step_count},
    multiplied by 100 to be a percentage.
    If no people have reached step {from_step} in the period, {conversion_rate} is zero.
    """
    def __init__(