Telemetry crash with TypeError '<' not supported between instances of 'NoneType' and 'int'

[leplatrem]

TypeError: '<' not supported between instances of 'NoneType' and 'int'
  File "__init__.py", line 82, in _finish
    finisher()
  File "transaction/_manager.py", line 255, in commit
    return self.manager.commit()
  File "transaction/_manager.py", line 132, in commit
    return self.get().commit()
  File "transaction/_transaction.py", line 267, in commit
    self._callBeforeCommitHooks()
  File "transaction/_transaction.py", line 333, in _callBeforeCommitHooks
    self._call_hooks(self._before_commit)
  File "transaction/_transaction.py", line 372, in _call_hooks
    hook(*(prefix_args + args), **kws)
  File "kinto_remote_settings/signer/listeners.py", line 232, in send_signer_events
    event.request.registry.notify(review_event)
  File "pyramid/registry.py", line 103, in notify
    [_ for _ in self.subscribers(events, None)]
  File "/opt/.venv/lib/python3.12/site-packages/zope/interface/registry.py", line 458, in subscribers
    return self.adapters.subscribers(objects, provided)
  File "/opt/.venv/lib/python3.12/site-packages/zope/interface/adapter.py", line 918, in subscribers
    subscription(*objects)
  File "pyramid/config/adapters.py", line 97, in derived_subscriber
    return subscriber(arg[0])
  File "kinto_remote_settings/signer/__init__.py", line 35, in on_review_approved
    metrics_service.count("plugins.signer.approved_changes", count)
  File "kinto/plugins/prometheus.py", line 147, in count
    m.inc(count)
  File "prometheus_client/metrics.py", line 312, in inc
    if amount < 0: