message: nydus-image: fix inspect format/command error
  This commit fixes two issues:
  1. In the prompt if users type enter key, the command
  will panic.
  2. Fix some indent issues.
diff: @@ -743,7 +743,10 @@ impl Executor {
  .strip_suffix("\n")
  .unwrap_or(&input)
  .split_ascii_whitespace();
  - let cmd = raw.next().unwrap();
  + let cmd = match raw.next() {
  + Some(c) => c,
  + None => return Ok(None),
  + };
  let args = raw.next().map(|a| a.trim());
  debug!("execute {:?} {:?}", cmd, args);
  