Allow to use run command without files for exec option

[rjwharry]

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`