Deletion of vulnerability-associated issuables prevents security report from loading

Summary

When an issue has been created from a security report, deleting that issue will prevent the Security report from rendering within the Pipeline view.

I have not yet tested if this is the case for issues created from the security dashboard, but expect the same

Steps to reproduce

1. Run pipeline on project with vulnerability; i.e. https://gitlab.com/gitlab-org/security-products/demos/yarn-remediation/pipelines/44238707/security.
2. Select vulnerability under "Security" tab
3. Click "Create Issue" to create issue from vulnerability
4. Go to "Edit" on issue page
5. Delete issue
6. Navigate back to pipeline page and see error Dependency scanning: Loading resulted in an error

Example Project

https://gitlab.com/gitlab-org/security-products/demos/yarn-remediation/pipelines/44238707/security

What is the current bug behavior?

Error is shown on Security tab when linked issue gets deleted

What is the expected correct behavior?

• Security tab should render regardless of whether an issue is associated
• Issues should be re-creatable if deleted ("Create Issue") should reappear

Relevant logs and/or screenshots

Output of checks

This is not present on Gitlab.com as it will ship with 11.7

Possible fixes

• Vulnerabilities::FeedbackEntity should handle missing issues and merge_requests.
• When issues or merge_requests are deleted associated vulnerabilities are disassociated