Investigate and find the reason why vulnerability_id is nil in some vulnerability findings

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Context: #541995 (comment 2519808338)

No vulnerability associated with a finding can cause issues further downstream:

1. vulnerability_id was nil on some findings
2. This lead to identifiers being nil
3. we return [] for identifiers
4. A policy failure happens:
   • undefined method project' for nil:Representation::VulnerabilityScannerEntry (NoMethodError)`
   • sentry error: GITLABCOM-8M3E

Internal data: #541995 (comment 2521085639)