Fix vulnerability attributes access when nil

What does this MR do and why?

Relates to #399117 (closed) and !123965 (merged)

Fixes https://sentry.gitlab.net/gitlab/gitlabcom/issues/4169293/?query=is:unresolved

The MR !123965 (merged) has been reverted with !126835 (merged), but we should include this fix when reintroducing the feature.

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