Fix Linear::UnboundedSearch error

What does this MR do and why?

This MR relates to: #419492

It fixes the error when accessing a project or group where parent_id is present, but parent is not.

Corresponding sentry errors:

• https://new-sentry.gitlab.net/organizations/gitlab/issues/1037686/?project=3
• https://new-sentry.gitlab.net/organizations/gitlab/issues/1037632/?project=3
• https://new-sentry.gitlab.net/organizations/gitlab/issues/1037671/?project=3

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Checkout the branch bojan/fix-error-500-orphaned-data
2. Create some nested groups/projects (e.g.: group1/group2/group3...)
3. Destroy a parent group (e.g.: group2)
4. You might need to temporarily disable the fk_7f813d8c90 CONSTRAINT when deleting the parent group
5. Verify that you can access the group3 by full URL 🎉