## 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
return {
            "bins": [(bin_from_seconds, person_count) for bin_from_seconds, person_count, _ in results],
            "average_conversion_time": results[0][2],
        }
    def get_query(self) -> str:
        steps_per_person_query = self.funnel_order.get_step_counts_query()
        self.params.update(self.funnel_order.params)
        
        
        from_step = self._filter.funnel_from_step or 0
        
        to_step = self._filter.funnel_to_step or len(self._filter.entities) - 1
        
        bin_count = self._filter.bin_count
        if bin_count is not None:
            
            if bin_count < 1:
                bin_count = 1
            elif bin_count > 90:
                bin_count = 90
            bin_count_identifier = str(bin_count)
            bin_count_expression = None
        else:
            
            bin_count_identifier = "bin_count"
            bin_count_expression = f"""
                count() AS sample_count,
                least(60, greatest(3, ceil(cbrt(sample_count)))) AS {bin_count_identifier},
            """