CommonAuditor: only print CustomEventType deprecation warning if Prop used

When doing a drop-and-reload of properties, the CommonAuditor deprecation warning for CustomEventTypes will always be printed, even if it's never actually set, and the size of the array is zero. This patch suppresses the warning in those cases.

Also, the original code overwrote the EventType property with CustomEventType, which would have removed any event types that were already set. We now print an error message if both are set.