Allow to use run command without files for exec option

\*Describe the bug\*When we use run command like this `backend.ai run --exec "python /home/work/test/hello-world.py" -v test python ./blank`, we have to upload at least one file even we don't need it.

\*To Reproduce\*Execute run command like this. `backend.ai run --exec "python /home/work/test/hello-world.py" -v test python`. Assume you have vfolder named "test" including "hello-world.py" file

\*Expected behavior\*Let user make a choice whether upload file or not. Above case, user can use run command without uploading file. `backend.ai run --exec "python /home/work/test/hello-world.py" -v test` python`

JIRA Issue: BA-249