Incident_key is nullable for events triggered by Stackdriver

[nick-fields]

Incidents triggered by Stackdriver GCP integration (using v1 events API) result in incident.* webhook events having a null incident_key.

It's unclear if this schema definition needs to be updated, if there is bug in the events being emitted for this webhook, or if there is another cause of null incident_key in this scenario.

This PR either fixes the schema or starts the conversation and hopefully leads to an answer