Resolve ActionView::Template::Error in Projects::Security::VulnerabilitiesController#create_issue

!40958 (merged) introduced a regression in creating and Issue from Vulnerability

Sentry issue: https://sentry.gitlab.net/gitlab/staginggitlabcom/issues/1846106

The cause of the issue is that Vulnerabilities::Finding#raw_metadata can have a key remediations which will have a value of [null] which will get serialized to [nil]. This gets passed to ee/app/views/vulnerabilities/issue_description.md.erb which has only a if vulnerability.remediations.present? check and then tries to use :diff key.

Related to #223142 (closed) and #214474 (closed)