Sendgrid API calls raising HTTP 400 Bad Requests

Sentry Issue: [FLAGSMITH-API-4XZ](https://flagsmith.sentry.io/issues/5746815615/?referrer=github_integration)

```
BadRequestsError: HTTP Error 400: Bad Request
(3 additional frame(s) were not displayed)
...
File "organisations/tasks.py", line 144, in handle_api_usage_notifications
handle_api_usage_notification_for_organisation(organisation)
File "organisations/task_helpers.py", line 182, in handle_api_usage_notification_for_organisation
_send_api_usage_notification(organisation, matched_threshold)
File "organisations/task_helpers.py", line 86, in _send_api_usage_notification
send_mail(

Error processing api usage for organisation REDACTED
```