ubusd: handle invoke on built-in event object without data

When a built-in object is invoked with UBUS_ATTR_DATA absent, recv_msg will be called with NULL as the msg argument and ubusd_forward_event and ubusd_alloc_event_pattern need to handle this. Otherwise, a truncated invoke of "send" or "register" on UBUS_SYSTEM_OBJECT_EVENT that is missing UBUS_ATTR_DATA will cause ubusd to crash with SIGSEGV.