Cron monitor shows "invalid duration" error when no duration is sent in check-in

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

1. Create a cron monitor with a schedule (e.g., every 10 minutes)
2. Send heartbeat check-ins using the simple URL format:
GET https://{ORG}.ingest.us.sentry.io/api/{PROJECT_ID}/cron/{MONITOR_SLUG}/{DSN_KEY}/?status=ok
3. Do NOT include any duration parameter in the request
4. Observe the monitor details page after some time

See Linear thread for Zendesk ticket.

### Expected Result

Check-ins should be recorded successfully without duration (since none was sent). No ingestion errors should appear.

### Actual Result

The monitor shows an ingestion error:
"Errors were encountered while ingesting check-ins for this monitor"
- "Invalid duration"
- "A check-in was sent with an invalid duration of '6839405000'"

The duration column shows "—" for affected check-ins. User reports this has occurred on two separate monitors and was never seen before.

### Product Area

Crons

### Link

_No response_

### DSN

_No response_

### Version

_No response_

Start at the cron check-in ingestion entry point for the simple URL shown in the reproduction steps, then reproduce a request without a duration and inspect how the recorded duration is derived. Confirm the fix by verifying that the check-in is accepted without an ingestion error and that its duration displays as "—".

Written by the indexing model from the issue text.