Handle events without 'object_attributes'

What does this MR do and why?

This fixes https://sentry.gitlab.net/gitlab/triage-ops/issues/4095617/?environment=production since our uptime ping's payload don't include object_attributes.

I'm updating the payload in https://gitlab.com/gitlab-org/quality/engineering-productivity-infrastructure/-/merge_requests/312 but I still think it will be more robust to also fix it here.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review