ExportCsvWorker fails if there are LabelLinks pointing to orphaned Labels

https://sentry.gitlap.com/gitlab/gitlabcom/issues/219707/

ArgumentError: comparison of String with nil failed
  issues/export_csv_service.rb:48:in `sort'
    'Labels' => -> (issue) { @labels[issue.id].sort.join(',').presence },
  issues/export_csv_service.rb:48:in `block in header_to_value_hash'
    'Labels' => -> (issue) { @labels[issue.id].sort.join(',').presence },
  csv_builder.rb:80:in `block in row'
    excel_sanitize(attribute.call(object))
  csv_builder.rb:78:in `map'
    attributes.map do |attribute|
  csv_builder.rb:78:in `row'
    attributes.map do |attribute|
...
(49 additional frame(s) were not displayed)

comparison of String with nil failed