## 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
from django_filters.rest_framework import DjangoFilterBackend
from drf_spectacular.utils import OpenApiResponse
from rest_framework import exceptions, request, serializers, status, viewsets
from rest_framework.decorators import action
from rest_framework.exceptions import PermissionDenied
from rest_framework.permissions import IsAuthenticated
from rest_framework.response import Response
from rest_framework.settings import api_settings
from rest_framework_csv import renderers as csvrenderers
from sentry_sdk import capture_exception
from statshog.defaults.django import statsd
from posthog.api.documentation import extend_schema
from posthog.api.forbid_destroy_model import ForbidDestroyModel
from posthog.api.insight_serializers import (
    FunnelSerializer,
    FunnelStepsResultsSerializer,
    TrendResultsSerializer,
    TrendSerializer,
)
from posthog.api.routing import StructuredViewSetMixin
from posthog.api.shared import UserBasicSerializer
from posthog.api.tagged_item import TaggedItemSerializerMixin, TaggedItemViewSetMixin
from posthog.api.utils import format_paginated_url
from posthog.client import sync_execute
from posthog.constants import (
    BREAKDOWN_VALUES_LIMIT,
    INSIGHT,
    INSIGHT_FUNNELS,
    INSIGHT_PATHS,
    INSIGHT_STICKINESS,