Documentation for nullable fields in Event Types in the Events API?

Hi Folks,

I am currently writing some integrations with the Events API. One thing that I’m not clear on is what types are nullable. For example, in the first CreateEvent for a repository the ref in the CreateEvent's payload will be null. Another example, the description field in a CreateEvent can be null if unset, instead of the empty string.

Is this documented anywhere? It does not seem to be mentioned here and it also doesn’t seem to be mentioned in the API spec - the spec does not look like it contains payload types yet, but I may have overlooked it.