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_