Backport of 'Add null user guard to adjourned group deletion service'

What does this MR do and why?

The adjourned group deletion service did not handle the case where current_user is nil, which could occur when a user has been deleted. This adds an early return in the authorization check and moves the deleted user shared example into the common shared examples so it covers both project and group adjourned deletion.

The shared example now also verifies that state_metadata and deletion_scheduled_by_user_id are cleared on restore. The previous group_deletion_schedule relied on a foreign key that cascaded on user deletion, which is not the case when using namespace state metadata.

See Add null user guard to adjourned group deletion... (!229017 - merged).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.