## 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
result = serializers.SerializerMethodField()
    last_refresh = serializers.SerializerMethodField(
        read_only=True,
        help_text="""
    The datetime this insight's results were generated.
    If added to one or more dashboards the insight can be refreshed separately on each.
    Returns the appropriate last_refresh datetime for the context the insight is viewed in
    (see from_dashboard query parameter).
    """,
    )
    created_by = UserBasicSerializer(read_only=True)
    last_modified_by = UserBasicSerializer(read_only=True)
    effective_privilege_level = serializers.SerializerMethodField()
    timezone = serializers.SerializerMethodField(help_text="The timezone this chart is displayed in.")
    dashboards = serializers.PrimaryKeyRelatedField(
        help_text="A dashboard ID for each of the dashboards that this insight is displayed on.",
        many=True,
        required=False,
        queryset=Dashboard.objects.all(),
    )
    filters_hash = serializers.CharField(
        read_only=True,
        help_text="""A hash of the filters that generate this insight.
        Used as a cache key for this result.
        A different hash will be returned if loading the insight on a dashboard that has filters.""",
    )
    class Meta: