Dear SQL Developer Dev Folks,
I've managed to solve the above dataloading issue by downgrading my SQL Developer from V184.108.40.206, MAIN-20.78 to V220.127.116.11 MAIN-16.84, i.e. the SQL*Loader doesn't meet unexpected literal "null" in my .ctl file generated by "sdcli cart cloud ..." command like above situation any more.
Hopefully the issue can be tracked on your side and get fixed soon.
remember, if you want to report bugs, best to open a Service Request with My Oracle Support