batches: don't panic on unexpected GitHub PR action

Created by: LawnGnome

When converting a raw payload to changeset event kind, we need to be able to handle payloads of unexpected types, especially nil, since we don't control the inputs that come in via webhooks.

The specific case that triggered this was a GitHub pull request event that had an unknown action: this results in a nil event keyer being returned, and that caused a panic in ChangesetEventKindFor().