dog/common: check empty argument for commands those need arguments

Advised by Hitoshi:

If we execute a command like this: dog node md plug "",
sheep dies because sheep daemon assumes a path name passed
by dog process is correct.

Response by Ruoyu:

I think the above checking should be processed in framework, too.
Otherwise, we must check it in every function that needs arguments.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>