Fix subscription seats update when namespace is absent

What does this MR do?

Gitlab subscription seat update worker would fail partially, if one subscription is without namespace. We skip this so the rest of the subscriptions will still be updated.

Sentry is still reported to detect similar bugs in the future.

A follow up issue is created for further investigation on why this data inconsistency occurs (#249083 (closed)).

Closes #249040 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.