Set `raw_event_t::arg_type` to `MTR_ARG_TYPE_NONE` 
Set `raw_event_t::arg_type` to `MTR_ARG_TYPE_NONE` 

See https://github.com/hrydgard/minitrace/issues/9 for context of why I did this
hrydgard/minitrace,hrydgard/minitrace