The root issue here is that we had been using `None` as a way of
encoding `event_types` as being an argument to not pass to the server
in the API codebase, but the marshalling to send this over the wire
didn't handle that possibility correctly.
This was incorrectly "fixed" in
409bb587429ec4dcb1220a8ed85ec1618ffde0ed; the root cause of the issue
was the refactor to the new approach for registering API endpoints.
d40328a311