Fix a case when MergeRequest are created with nil as attributes

What does this MR do and why?

When a pipeline for a stable branch failed, we created a MergeRequest object with nil given as initial attributes, which resulted in https://sentry.gitlab.net/gitlab/triage-ops/issues/4166853/?environment=production.

This merge request ensures that if given attributes are nil, we just return and don't create a MergeRequest object.

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