Fix pipeline of vulnerability location link

What does this MR do and why?

Failed job: https://jihulab.com/gitlab-cn/gitlab/-/jobs/10889294

ee/spec/services/vulnerability_exports/export_service_spec.rb:133

1) VulnerabilityExports::ExportService#export when the vulnerability_export is in `created` state when the export format is csv when the exportable object is a group finds all vulnerabilities for the group
     Failure/Error: yield relation
     
     TypeError:
       no implicit conversion of nil into String
     # ./ee/app/models/ee/vulnerability.rb:225:in `join'
     # ./ee/app/models/ee/vulnerability.rb:225:in `blob_path'
     # ./jh/app/services/jh/vulnerability_exports/exporters/csv_service.rb:15:in `block in <module:CsvService>'

Root cause

# https://jihulab.com/gitlab-cn/gitlab/blob/eb588e271b513e374188b70b1e3de7cd2e1915df/ee/app/models/ee/vulnerability.rb#L225

::Gitlab::Routing.url_helpers.project_blob_path(project, File.join(finding.sha, finding_file))

# https://jihulab.com/gitlab-cn/gitlab/blob/c76a6740e01ae2b437c4bed6c7f6cf49ff6e8cd7/ee/app/models/vulnerabilities/finding.rb#L148

    commit_sha = @sha || pipeline_branch

# https://jihulab.com/gitlab-cn/gitlab/blob/c76a6740e01ae2b437c4bed6c7f6cf49ff6e8cd7/ee/app/models/vulnerabilities/finding.rb#L409
    def pipeline_branch
      last_finding_pipeline&.sha || project.default_branch
    end

# https://jihulab.com/gitlab-cn/gitlab/blob/814b91be037d9f95c5a3b119be4cb673550a3b73/app/models/concerns/has_repository.rb#L74

  def default_branch
    @default_branch ||= repository.empty? ? default_branch_from_preferences : repository.root_ref
  end

Test project is an empty project, and test env have no default branch settings, so, finding.sha returns nil, breaks the join.

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