Fix FK error for DumpWriteBufferCronWorker

Pavel Shutsinrequested to merge
569128-usageevents-dumpwritebuffercronworker-throws-activerecord-invalidforeignkey-when-namespace-was-deleted into master

What does this MR do and why?

Usage events should be considered invalid if provided namespace does not exist in DB anymore.

MR acceptance checklist

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

Related to #569128 (closed)

Merge request reports