There isn’t any documentation in NW help about null handling modes for events. In endpoint definitions, there are three modes - Use Null, Use Sample Payload Value, and Remove Key and Value. What does the event payload pass through when the value is null?
Generally it just omits the field entirely. The exception to this is within a logic block, if you clear a field before queueing the event with that record, that field will be sent as a null value rather than omitted. If your event goes to an external target, nulls are always omitted to save payload space.