## 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/test/test_funnel_time_to_convert.py
"funnel_to_step": 1,
                "funnel_window_days": 7,
                "bin_count": 7,
                "events": [
                    {"id": "step one", "order": 0},
                    {"id": "step two", "order": 1},
                    {"id": "step three", "order": 2},
                ],
            }
        )
        funnel_trends = ClickhouseFunnelTimeToConvert(filter, self.team, ClickhouseFunnel)
        results = funnel_trends.run()
        
        self.assertEqual(
            results,
            {
                "bins": [
                    (2220.0, 2),  
                    (13732.0, 0),  
                    (25244.0, 0),  
                    (36756.0, 0),
                    (48268.0, 0),
                    (59780.0, 0),
                    (71292.0, 1),  
                    (82804.0, 0),
                ],
                "average_conversion_time": 29_540,
            },
        )
    def test_auto_bin_count_total(self):