Code quality reports: ArgumentError: comparison of NilClass with 1 failed

https://sentry.gitlab.net/gitlab/gitlabcom/issues/3091804/?referrer=gitlab_plugin

ArgumentError: comparison of NilClass with 1 failed
  lib/gitlab/ci/reports/codequality_reports.rb:34:in `sort_by'
    @degradations = @degradations.sort_by do |_fingerprint, degradation|
  lib/gitlab/ci/reports/codequality_reports.rb:34:in `sort_degradations!'
    @degradations = @degradations.sort_by do |_fingerprint, degradation|
  lib/gitlab/ci/reports/codequality_reports_comparer.rb:13:in `initialize'
    @head_report.sort_degradations!
  app/services/ci/compare_reports_base_service.rb:37:in `new'
    comparer_class.new(base_report, head_report)
  app/services/ci/compare_reports_base_service.rb:37:in `build_comparer'
    comparer_class.new(base_report, head_report)
...
(99 additional frame(s) were not displayed)

comparison of NilClass with 1 failed