## Bug description

The new funnels time conversion endpoint sometimes returns bins with negative time values. This makes it difficult to reflect accurate time conversion funnels data on the frontend. @Twixes @neilkakkar 

<img width="379" alt="Screen Shot 2021-07-13 at 9 14 42 PM" src="https://user-images.githubusercontent.com/13460330/125560350-3ef2eb03-5fc5-4c89-a0ab-988c945d7d87.png">

Example CH localhost [link ](http://localhost:8000/insights?insight=FUNNELS&properties=%5B%5D&filter_test_accounts=false&interval=day&events=%5B%7B%22id%22%3A%22%24pageview%22%2C%22name%22%3A%22%24pageview%22%2C%22type%22%3A%22events%22%2C%22order%22%3A0%7D%2C%7B%22id%22%3A%22%24pageview%22%2C%22name%22%3A%22%24pageview%22%2C%22type%22%3A%22events%22%2C%22order%22%3A1%7D%2C%7B%22id%22%3A%22%24pageview%22%2C%22name%22%3A%22%24pageview%22%2C%22type%22%3A%22events%22%2C%22order%22%3A2%7D%5D&display=FunnelsTimeToConvert&actions=%5B%5D&new_entity=%5B%5D) to reproduce bug.

Request Payload

<img width="612" alt="Screen Shot 2021-07-13 at 9 19 06 PM" src="https://user-images.githubusercontent.com/13460330/125560592-9e2a0670-606c-485a-a102-7e085c72f225.png">


## Expected behavior

No negative time bin values. 

## How to reproduce

1. Navigate to new funnels time conversion page. Make sure to turn on this FF: `4535-funnel-bar-viz`
2. Add a few identical actions/events
3. Open up network requests to see the response to `api/funnels`

## Environment

- [X] PostHog Cloud
- [ ] self-hosted PostHog, version/commit: _please provide_

#### *Thank you* for your bug report – we love squashing them!

ee/clickhouse/queries/funnels/funnel_time_to_convert.py
f"( SELECT {bin_count_identifier} FROM histogram_params ) AS {bin_count_identifier},"
                    if bin_count_expression else ""
                }
                ( SELECT from_seconds FROM histogram_params ) AS histogram_from_seconds,
                ( SELECT to_seconds FROM histogram_params ) AS histogram_to_seconds,
                ( SELECT average_conversion_time FROM histogram_params ) AS histogram_average_conversion_time
            SELECT
                bin_from_seconds,
                person_count,
                histogram_average_conversion_time AS average_conversion_time
            FROM (
                -- Calculating bins from step runs
                SELECT
                    histogram_from_seconds + floor(({steps_average_conversion_time_expression_sum} - histogram_from_seconds) / bin_width_seconds) * bin_width_seconds AS bin_from_seconds,
                    count() AS person_count
                FROM step_runs
                -- We only need to check step to_step here, because it depends on all the other ones being NOT NULL too
                WHERE step_{to_step}_average_conversion_time_inner IS NOT NULL
                GROUP BY bin_from_seconds
            ) results
            FULL OUTER JOIN (
                -- Making sure bin_count bins are returned
                -- Those not present in the results query due to lack of data simply get person_count 0
                SELECT histogram_from_seconds + number * bin_width_seconds AS bin_from_seconds FROM system.numbers LIMIT {bin_count_identifier} + 1
            ) fill
            USING (bin_from_seconds)
            ORDER BY bin_from_seconds
            SETTINGS allow_experimental_window_functions = 1"""
        return query