## Bug description

From [a customer report](https://posthoghelp.zendesk.com/agent/tickets/653):

```
I'm accessing insight results through the API, and I’ve noticed that sometimes the result is null in the response.
When I go and view the insight in Posthog and tap “refresh” the result starts to return in the API response.
I’ve also noticed there is a “refreshing” boolean flag in the insight response.
How do I set the insight to either refresh periodically or on request so I always get a result through the API?
```

It seems like the insight returns null if it's not refreshed manually from the frontend.

## How to reproduce

1. Call an insight with GET | /api/projects/:project_id/insights/:id
2. Response is `null`
3. Refresh the insight from the frontend
4. Response is populated

## Environment

- [ ] PostHog Cloud
- [ ] self-hosted PostHog, version/commit: please provide

## Additional context


posthog/api/insight.py
request = self.context["request"]
        team = Team.objects.get(id=self.context["team_id"])
        tags = validated_data.pop("tags", None)  
        created_by = validated_data.pop("created_by", request.user)
        dashboards = validated_data.pop("dashboards", None)
        insight = Insight.objects.create(
            team=team, created_by=created_by, last_modified_by=request.user, **validated_data
        )
        if dashboards is not None:
            for dashboard in Dashboard.objects.filter(id__in=[d.id for d in dashboards]).all():
                if dashboard.team != insight.team:
                    raise serializers.ValidationError("Dashboard not found")
                DashboardTile.objects.create(insight=insight, dashboard=dashboard, last_refresh=now())
                insight.last_refresh = now()  
        
        self._attempt_set_tags(tags, insight)
        log_insight_activity(
            activity="created",
            insight=insight,
            insight_id=insight.id,
            insight_short_id=insight.short_id,
            organization_id=self.context["request"].user.current_organization_id,
            team_id=team.id,
            user=self.context["request"].user,
        )
        return insight
    def update(self, instance: Insight, validated_data: Dict, **kwargs) -> Insight: