Guard against empty entity_types in S3 StreamingDestinations

What does this MR do and why?

Guard against empty entity_types

• The database doesn't let these exist anymore, but it seems there are clearly still variants in the data based on Sentry errors.

References

• Refs: https://new-sentry.gitlab.net/organizations/gitlab/issues/1520856/?query=StreamingWorker&referrer=issue-stream&statsPeriod=14d&stream_index=7

How to set up and validate locally

0. Not possible to do in the UI
1. You'd have to do the same thing that happens in the changed spec in rails console.

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