Fix nil error when checking checkpoints that are not fully instantiated (19-0 backport)

What does this MR do and why?

Backport of !243734 (merged) to 19-0-stable-ee. Cherry-pick had a small conflict resolved.

Fresh 19.0+ Ultimate instances hit NoMethodError: undefined method '>=' for nil in PackageMetadata::AdvisoriesSyncWorker and LicensesSyncWorker because SyncService gets an unpersisted checkpoint (sequence: nil) from find_or_initialize_by that passes the existing checkpoint.blank? guard, and the label sync path then blows up in caught_up? before listObjects is reached. Advisory and license data never ingest and Dependency Scanning v2 fails at vuln-scan with advisory data is missing from your GitLab instance.

The one-liner extends the guard so an unpersisted-with-nil-sequence checkpoint takes the listobjects_blank_checkpoint fallback.

Conflict resolution notes

19.0 does not yet have track_sync_path / internal-events tracking in Connector::Gcp (introduced later on master). The cherry-pick conflicted on data_after because master's version tracks an internal event before returning; 19.0's shape is return all_files if checkpoint.blank?. I kept 19.0's one-line style and extended the guard: return all_files if checkpoint.blank? || checkpoint.sequence.blank?. I also dropped the it 'tracks the listobjects_blank_checkpoint event' spec from the backport since it asserts on an event the 19.0 code doesn't fire. The it 'falls back to listObjects instead of raising' spec is kept and covers the actual bug.

Relates to #603943 (closed).

Draft until !243734 (merged) merges to master and deploys to gitlab.com.

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, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

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.